What you want to do

Creating a class that does this

--Creating a crosshair that follows the cursor --Display x, y coordinates on the label --Label color will be the same as the plot line --None display if out of y-axis range

environment

Mac OS Python 3.8.5

PyQt5 5.15.2 PyQt5-sip 12.8.1 pyqtgraph 0.11.1

pip install PyQt5 PyQt5-sip pyqtgraph

pyqtgraph.exsamples

import pyqtgraph.examples as ex
ex.run()

You can see various sample graphs at. I referred to Crosshair / Mouse interaction this time.

What I made

import dataclasses
from typing import Union
import sys

from PyQt5 import QtWidgets
import pyqtgraph as pg


@dataclasses.dataclass
class CursorCrossHairLabel(object):
    """A class that creates a crosshair that follows the cursor and adds it to the graph
Display coordinates if the cursor is within coordinates
    Attributes #
    ----------
    plotter: pg.PlotItem
Graph to add a crosshair
    label: pg.LabelItem
Label displaying coordinates
    vertical_line: pg.InfiniteLine
Vertical line chasing the cursor
    horizontal_line: pg.InfiniteLine
Horizontal line chasing the cursor
    proxy: pg.SignalProxy
        self.cursor_Object that controls the execution interval of moved
    y_label: str default='y'
string of y-value legend
    label_font_size: Union[int, float] default=14
Label font size
    digit :int default=3
Specify display of coordinates after the decimal point

    Examples
    ----------
    import dataclasses
    from typing import Union
    import sys

    from PyQt5 import QtWidgets
    import pyqtgraph as pg

    app = QtWidgets.QApplication(sys.argv)
    graph_widget = pg.GraphicsLayoutWidget()

    label = pg.LabelItem(justify='right')
    graph_widget.addItem(label)

    plotter = graph_widget.addPlot(row=0, col=0)
    plotter.showGrid(x=True, y=True, alpha=1)

    curve1 = pg.PlotCurveItem([i ** 2 for i in range(10)], pen=pg.mkPen('#f00'))
    curve2= pg.PlotCurveItem([2*i ** 2 for i in range(10)], pen=pg.mkPen('#0f0'))
    plotter.addItem(curve1)
    plotter.addItem(curve2)

    cross_hair = CursorCrossHairLabel(plotter, label)

    graph_widget.show()
    sys.exit(app.exec_())
    """
    plotter: pg.PlotItem
    label: pg.LabelItem
    y_label: str = 'y'
    label_font_size: Union[int, float] = 14
    digit: int = 3
    v_color: dataclasses.InitVar[str] = '#ffd700'
    v_width: dataclasses.InitVar[Union[int, float]] = 2
    h_color: dataclasses.InitVar[str] = '#ffd700'
    h_width: dataclasses.InitVar[Union[int, float]] = 2

    def __post_init__(self, v_color: str, v_width: Union[int, float], h_color: str, h_width: Union[int, float]) -> None:
        """Continuation of constructor
        Parameters
        ----------
        v_color: str default='#ffd700'
Vertical line color
        v_width: Union[int, float] default=2
Vertical line width
        h_color: str default='#ffd700'
Horizontal line color
        h_width: Union[int, float] default=2
Horizontal line width
        """
        self.vertical_line = pg.InfiniteLine(angle=90, movable=False, pen=pg.mkPen(v_color, width=v_width))
        self.horizontal_line = pg.InfiniteLine(angle=0, movable=False, pen=pg.mkPen(h_color, width=h_width))
        #Added to plotter
        self.plotter.addItem(self.vertical_line, ignoreBounds=True)
        self.plotter.addItem(self.horizontal_line, ignoreBounds=True)

        self.proxy = pg.SignalProxy(self.plotter.scene().sigMouseMoved, rateLimit=60, slot=self.cursor_moved)

        self.set_label()

    def set_label(self):
        """Initial values of coordinates(None)Set on the label"""
        coord_text = f"<span style='font-size: {self.label_font_size}pt'>x=None<br>"
        for y_index, curve in enumerate(self.plotter.curves, 1):
            curve_color: str = curve.opts['pen'].color().name()
            coord_text += f" <span style='color: {curve_color}'>{self.y_label}{y_index}=None</span><br>"
        self.label.setText(coord_text)

    def cursor_moved(self, window_coord) -> None:
        """Crosshair movement and coordinates are displayed at the cursor position
Display x value if the cursor coordinates are in the graph
If there is data in the y coordinate of the cursor, the y value is displayed. If it is out of range, None is displayed.
y coordinate label is the same color as curve

        Parameters
        ----------
        window_coord: tuple
Screen pixel coordinates, ex) (PyQt5.QtCore.QPointF(2.0, 44.0),)

        Notes
        ----------
curve is pyqt graph.PlotCurveItem()If you do not create it with curve_An error occurs at the color part
        ex) OK
        import pyqtgraph as pg
        window = pg.PlotWidget()
        curve = pg.PlotCurveItem(pen=pg.mkPen('#ff0000))
        window.addItem(curve)

        ex) Error
        import pyqtgraph as pg
        window = pg.PlotWidget()
        curve = window.plot(pen=...)

Description of variables in function===================
        pos: PyQt5.QtCore.QPointF
In-screen coordinates of the cursor ex) PyQt5.QtCore.QPointF(2.0, 44.0)
        cursor_point: QtCore.QPointF
Coordinates in the cursor graph ex) PyQt5.QtCore.QPointF(269.0678171506131, 0.695852534562212)
        self.plotter.curves: List[pg.PlotCurveItem]
List of curves in plotter ex) [<pg.PlotCurveItem object at ...>, ...]
        curve_color: str
curve color ex) '#ff0000',It doesn't matter but curve.opts['pen'].color()Type is Qt Gui.QColor
        =================================
        """
        pos = window_coord[0]

        if self.plotter.sceneBoundingRect().contains(pos):
            cursor_point = self.plotter.vb.mapSceneToView(pos)
            x_cursor_point: float = cursor_point.x()
            coord_text = f"<span style='font-size: {self.label_font_size}pt'>x={x_cursor_point:.{self.digit}f}<br>"
            data_index = int(x_cursor_point)

            for label_index, curve in enumerate(self.plotter.curves, 1):
                curve_color = curve.opts['pen'].color().name()

                if len(curve.yData) != 0 and 0 <= data_index < len(curve.yData):
                    y_value = curve.yData[data_index]
                    coord_text += f" <span style='color: {curve_color}'>" \
                                  f"{self.y_label}{label_index}={y_value:.{self.digit}f}</span><br>"
                else:
                    coord_text += f" <span style='color: {curve_color}'>{self.y_label}{label_index}=None</span><br>"
            self.label.setText(coord_text)
            #Cross line movement
            self.vertical_line.setPos(cursor_point.x())
            self.horizontal_line.setPos(cursor_point.y())

argument

Required arguments

--plotter: pyqtgraph.PlotItem, a graph that displays a crosshair --label: pyqtgraph.LabelItem, label to display coordinates

Other

--y_label: str, change legend character for y value, default ='y' --label_font_size: Union [int, float], label font size, default = 14 --digit: int Specify decimal point display of coordinates, default = 3

Also, the color and width of the cross line

Example of use

The execution result is the above gif

import dataclasses
from typing import Union
import sys

import numpy as np
from PyQt5 import QtWidgets
import pyqtgraph as pg

app = QtWidgets.QApplication(sys.argv)

graph_widget = pg.GraphicsLayoutWidget()

label = pg.LabelItem(justify='right')
graph_widget.addItem(label)

plotter = graph_widget.addPlot(row=0, col=0)
plotter.showGrid(x=True, y=True, alpha=1)

data = np.linspace(0, 100, 1000)

curve = pg.PlotCurveItem(data, pen=pg.mkPen('#f00', width=3))
curve2 = pg.PlotCurveItem(data * 2, pen=pg.mkPen('#0f0', width=3))
curve3 = pg.PlotCurveItem(data * 3, pen=pg.mkPen('#00f', width=3))
plotter.addItem(curve)
plotter.addItem(curve2)
plotter.addItem(curve3)
#this
cross_hair = CursorCrossHairLabel(plotter, label, y_label='sample', label_font_size=18)

graph_widget.show()
sys.exit(app.exec_())

When you don't want the label and graph to overlap

graph_widget = pg.GraphicsLayoutWidget()

plotter = graph_widget.addPlot()
plotter.showGrid(x=True, y=True, alpha=1)

label = pg.LabelItem(justify='right')
graph_widget.addItem(label)

Labelless ver

import dataclasses
from typing import Union
import sys

from PyQt5 import QtWidgets
import pyqtgraph as pg


@dataclasses.dataclass
class CursorCrossHair(object):
    """A class that creates a crosshair that follows the cursor and adds it to the graph
    Attributes #
    ----------
    plotter: pg.PlotItem
Graph to add a crosshair
    vertical_line: pg.InfiniteLine
Vertical line chasing the cursor
    horizontal_line: pg.InfiniteLine
Horizontal line chasing the cursor
    proxy: pg.SignalProxy
        self.cursor_Object that controls the execution interval of moved
    
    Examples
    ----------
    import dataclasses
    from typing import Union
    import sys

    from PyQt5 import QtWidgets
    import pyqtgraph as pg

    app = QtWidgets.QApplication(sys.argv)
    graph_widget = pg.GraphicsLayoutWidget()

    plotter = graph_widget.addPlot(row=0, col=0)
    plotter.showGrid(x=True, y=True, alpha=1)

    curve1 = pg.PlotCurveItem([i ** 2 for i in range(10)], pen=pg.mkPen('#f00'))
    curve2= pg.PlotCurveItem([2*i ** 2 for i in range(10)], pen=pg.mkPen('#0f0'))
    plotter.addItem(curve1)
    plotter.addItem(curve2)

    cross_hair = CursorCrossHair(plotter)

    graph_widget.show()
    sys.exit(app.exec_())
    """
    plotter: pg.PlotItem
    v_color: dataclasses.InitVar[str] = '#ffd700'
    v_width: dataclasses.InitVar[Union[int, float]] = 2
    h_color: dataclasses.InitVar[str] = '#ffd700'
    h_width: dataclasses.InitVar[Union[int, float]] = 2

    def __post_init__(self, v_color: str, v_width: Union[int, float], h_color: str, h_width: Union[int, float]) -> None:
        """Continuation of constructor
        Parameters
        ----------
        v_color: str default='#ffd700'
Vertical line color
        v_width: Union[int, float] default=2
Vertical line width
        h_color: str default='#ffd700'
Horizontal line color
        h_width: Union[int, float] default=2
Horizontal line width
        """
        self.vertical_line = pg.InfiniteLine(angle=90, movable=False, pen=pg.mkPen(v_color, width=v_width))
        self.horizontal_line = pg.InfiniteLine(angle=0, movable=False, pen=pg.mkPen(h_color, width=h_width))
        #Added to plotter
        self.plotter.addItem(self.vertical_line, ignoreBounds=True)
        self.plotter.addItem(self.horizontal_line, ignoreBounds=True)

        self.proxy = pg.SignalProxy(self.plotter.scene().sigMouseMoved, rateLimit=60, slot=self.cursor_moved)

    def cursor_moved(self, window_coord) -> None:
        """Move the crosshair to the cursor position
         Parameters
         ----------
         window_coord: tuple
Screen pixel coordinates, ex) (PyQt5.QtCore.QPointF(2.0, 44.0),)

         Notes
         ----------
Description of variables in function===================
         pos: PyQt5.QtCore.QPointF
In-screen coordinates of the cursor ex) PyQt5.QtCore.QPointF(2.0, 44.0)
         cursor_point: QtCore.QPointF
Coordinates in the cursor graph ex) PyQt5.QtCore.QPointF(269.0678171506131, 0.695852534562212)
         =================================
         """
        pos = window_coord[0]
        if self.plotter.sceneBoundingRect().contains(pos):
            cursor_point = self.plotter.vb.mapSceneToView(pos)
            self.vertical_line.setPos(cursor_point.x())
            self.horizontal_line.setPos(cursor_point.y())