pyqtgraph plot with x-Axis in date units

Question

I try to plot a chart with dates in the x-axis. But the units displayed are in hours and minutes.

import sys
import datetime
import pyqtgraph as pg
from PySide6.QtWidgets import QApplication, QMainWindow


class MainWindow(QMainWindow):
    def __init__(self):
        super().__init__()

        self.plotWidget = pg.PlotWidget()
        self.setCentralWidget(self.plotWidget)

        self.scatter = pg.ScatterPlotItem()
        self.plotWidget.addItem(self.scatter)
        self.plotWidget.setBackground('w')
        dateAxis = pg.DateAxisItem()
        self.plotWidget.setAxisItems({'bottom': dateAxis})

        self.add_points()

    def add_points(self):
        x_values = [datetime.date(2024, 1, 1), datetime.date(
            2024, 3, 1), datetime.date(2024, 4, 21)]
        points = [
            {'pos': (x_values[0].toordinal(), 1)},
            {'pos': (x_values[1].toordinal(), 2)},
            {'pos': (x_values[2].toordinal(), 3)}]
        self.scatter.addPoints(points)

if __name__ == '__main__':
    app = QApplication(sys.argv)
    window = MainWindow()
    window.show()
    sys.exit(app.exec())

How can I force the x-Axis unit with dates? eg 01.01.2024

Thanks for hints

Vik

Rik · Accepted Answer · 2024-04-21 20:56:58Z

0

This is the best I could get it:

import sys
import datetime
import pyqtgraph as pg
from PySide6.QtWidgets import QApplication, QMainWindow
import time
from collections import OrderedDict


class MainWindow(QMainWindow):
    def __init__(self):
        super().__init__()

        self.plotWidget = pg.PlotWidget()
        self.setCentralWidget(self.plotWidget)

        self.scatter = pg.ScatterPlotItem()
        self.plotWidget.addItem(self.scatter)
        self.plotWidget.setBackground('w')
        dateAxis = pg.DateAxisItem()
        new_zoom_levels = OrderedDict()
        for index, (key, item) in enumerate(dateAxis.zoomLevels.items()):
            if index == 1:
                new_zoom_levels[key*2] = item
            else:
                new_zoom_levels[key] = item

        dateAxis.zoomLevels = new_zoom_levels
        self.plotWidget.setAxisItems({'bottom': dateAxis})

        self.add_points()

    def add_points(self):
        x_values = [datetime.datetime(2024, 1, 1), datetime.datetime(
            2024, 3, 1), datetime.datetime(2024, 4, 21)]
        points = [
            {'pos': (x_values[0].timestamp(), 1)},
            {'pos': (x_values[1].timestamp(), 2)},
            {'pos': (x_values[2].timestamp(), 3)}]
        print(points)
        print(time.time())
        self.scatter.addPoints(points)

if __name__ == '__main__':
    app = QApplication(sys.argv)
    window = MainWindow()
    window.show()
    sys.exit(app.exec())

The biggest problem in your code is that pyqtgraph expects a unix timestamp, while in your code, an ordinal representation of a date is given (number of days since 1 January 1 AD). This is fixed by changing the datetime.date, to datetime.datetime objects, and then getting the unix timestamp by using .timestamp().

A problem that I ran into, when I did this is that the days where not visible, only the months. I ran into issues with the proper way (using dateAxis.setZoomLevelForDensity()), so I used a bit of a workaround, by adjusting dateAxis.ZoomLevels. If anyone can get the proper way working that would be nice.

answered Apr 21, 2024 at 20:56

Rik

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2 Comments

Vik Over a year ago

The solution works for me. When one zooms in, the days are shown too. What I don't understand is what the modified new_zoom_levels is for. I played around with it and have problems to understand the effect. Even when I entered an empty new_zoom_levels the results is good. A hint how to get a better understanding would be great. Thanks

Rik Over a year ago

@Vik the new_zoom_levels bit is a workaround to get the days to show instead of just the months. In the code for the dateAxis I found ZoomLevels which is a dictionary that defines when to switch to showing months on the x-axis, when days, when hours and so on. This is most definitely not the proper solution, but it's the only thing I could get working.

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