Remove / Delete Legend in PyQtGraph

Question

Two lines are created on the live chart and a legend is added. At the next update, the lines on the chart are deleted using self.pw.clear().

But the legend is not deleted, and with each update, a new instance of the legend is added, there are a lot of them and the FPS of the schedule update quickly drops.

Here http://www.pyqtgraph.org/documentation/graphicsItems/plotitem.html says Clear(): “Remove all items from the ViewBox.”

Attempting to Clear / removeItem - have not yet helped (either the syntax is incorrect, or the procedure call is incorrect).

How to delete a legend when updating a chart or stop multiple legends creation?

import random
from PyQt5 import QtGui
import pyqtgraph as pg
import sys

class Mainwindow(QtGui.QMainWindow):
    def __init__(self, parent):
        super(Mainwindow, self).__init__()
        self.centralWidget = QtGui.QWidget()
        self.setCentralWidget(self.centralWidget)
        self.resize(1000, 500)

        self.vbox = QtGui.QVBoxLayout()
        self.pw = pg.PlotWidget()
        self.vbox.addWidget(self.pw)
        self.centralWidget.setLayout(self.vbox)

        # Update chart
        self.timer = pg.QtCore.QTimer()
        self.timer.setSingleShot(False)
        self.timer.timeout.connect(self.update)
        self.timer.start(10)

    def update(self):
        x = []
        y = []
        z = []
        for i in range(10000):
            x.append(i)
            y.append(random.uniform(0, 1))
            z.append(1 + random.uniform(0, 1))

        self.pw.clear()
        line_red = pg.PlotCurveItem(x, y, clear=True, pen='r', name='Red')
        line_yellow = pg.PlotCurveItem(x, z, clear=True, pen='y', name='Yellow')
        self.pw.addItem(line_red)
        self.pw.addItem(line_yellow)
        self.pw.addLegend()

app = QtGui.QApplication(sys.argv)
ex = Mainwindow(app)
ex.show()
sys.exit(app.exec_())

score 2 · Accepted Answer · answered Mar 22 '20 at 15:24

_{You have an XY problem, instead of asking How to update the plot? questions How to eliminate the duplicate legend ?. So I will point out a solution to the underlying problem.}

Considering the above, the logic is to only create the items once and update the information using the setData() method.

import random
import sys

import pyqtgraph as pg
from pyqtgraph.Qt import QtGui


class Mainwindow(QtGui.QMainWindow):
    def __init__(self, parent):
        super(Mainwindow, self).__init__()
        self.centralWidget = QtGui.QWidget()
        self.setCentralWidget(self.centralWidget)
        self.resize(1000, 500)

        vbox = QtGui.QVBoxLayout(self.centralWidget)
        self.pw = pg.PlotWidget()
        self.pw.addLegend()
        vbox.addWidget(self.pw)

        # Update chart
        self.timer = pg.QtCore.QTimer()
        self.timer.setSingleShot(False)
        self.timer.timeout.connect(self.update)
        self.timer.start(10)

        self.line_red = pg.PlotCurveItem(clear=True, pen="r", name="Red")
        self.line_yellow = pg.PlotCurveItem(clear=True, pen="y", name="Yellow")

        self.pw.addItem(self.line_red)
        self.pw.addItem(self.line_yellow)

    def update(self):
        x = []
        y = []
        z = []
        for i in range(10000):
            x.append(i)
            y.append(random.uniform(0, 1))
            z.append(1 + random.uniform(0, 1))
        self.line_red.setData(x, y)
        self.line_yellow.setData(x, z)


if __name__ == "__main__":
    app = QtGui.QApplication(sys.argv)
    ex = Mainwindow(app)
    ex.show()
    sys.exit(app.exec_())

Yes, now I see my mistake. Your code is working correctly. Also, thanks to your decision with setData(), I also compared the performance of your code and mine (both without a legend) - your code added +14..19% to the FPS. Cool! — paradoxarium, Mar 22 '20 at 16:22

Remove / Delete Legend in PyQtGraph

1 Answers1