Show string values on x-axis in pyqtgraph

Question

I want to display string values for ticks on x-axis in pyqtgraph. Right now I am unable to figure out how to do that.

Ex:

x = ['a', 'b', 'c', 'd', 'e', 'f']
y = [1, 2, 3, 4, ,5, 6]
pg.plot(x, y) 

When I try to pass the string array to the x variable it tries converting that to float and breaks the GUI with the error message.

Answer 1

Usually in pyqtgraph when dealing with custom axis strings people subclass AxisItem and override tickStrings with the strings they want displayed.

See e.g. pyqtgraph : how to plot time series (date and time on the x axis)?

Pyqtgraphs axisitem also has a built in setTicks that allows you to specify the ticks that are going to be displayed, this could be done for a simple problem like this instead of subclassing the AxisItem.

---

Plotting with custom string on x-axis could be done like this.

• Create a dict with x-values together with the strings to be displayed on the axis.

xdict = {0:'a', 1:'b', 2:'c', 3:'d', 4:'e', 5:'f'}

or by using

x = ['a', 'b', 'c', 'd', 'e', 'f']
xdict = dict(enumerate(x))

• Use setTicks in an AxisItem or Subclass AxisItem and in tickStrings find the string corresponding to the value.

1. Using a standard pyqtgraph AxisItem and setTicks

    from PyQt4 import QtCore
    import pyqtgraph as pg

    x = ['a', 'b', 'c', 'd', 'e', 'f']
    y = [1, 2, 3, 4, 5, 6]
    xdict = dict(enumerate(x))

    win = pg.GraphicsWindow()
    stringaxis = pg.AxisItem(orientation='bottom')
    stringaxis.setTicks([xdict.items()])
    plot = win.addPlot(axisItems={'bottom': stringaxis})
    curve = plot.plot(list(xdict.keys()),y)

    if __name__ == '__main__':
        import sys
        if sys.flags.interactive != 1 or not hasattr(QtCore, 'PYQT_VERSION'):
            pg.QtGui.QApplication.exec_()

2. Implementation by subclassing AxisItem

This is a more generic method which can be easily changed to all kinds of fun things e.g. converting a unix timestamp to a date.

    from PyQt4 import QtCore
    import pyqtgraph as pg
    import numpy as np

    class MyStringAxis(pg.AxisItem):
        def __init__(self, xdict, *args, **kwargs):
            pg.AxisItem.__init__(self, *args, **kwargs)
            self.x_values = np.asarray(xdict.keys())
            self.x_strings = xdict.values()

        def tickStrings(self, values, scale, spacing):
            strings = []
            for v in values:
                # vs is the original tick value
                vs = v * scale
                # if we have vs in our values, show the string
                # otherwise show nothing
                if vs in self.x_values:
                    # Find the string with x_values closest to vs
                    vstr = self.x_strings[np.abs(self.x_values-vs).argmin()]
                else:
                    vstr = ""
                strings.append(vstr)
            return strings

    x = ['a', 'b', 'c', 'd', 'e', 'f']
    y = [1, 2, 3, 4, 5, 6]
    xdict = dict(enumerate(x))

    win = pg.GraphicsWindow()
    stringaxis = MyStringAxis(xdict, orientation='bottom')
    plot = win.addPlot(axisItems={'bottom': stringaxis})
    curve = plot.plot(list(xdict.keys()),y)

    if __name__ == '__main__':
        import sys
        if sys.flags.interactive != 1 or not hasattr(QtCore, 'PYQT_VERSION'):
            pg.QtGui.QApplication.exec_()

Screenshot from example:

Answer 2

I find it easiest to prepare a list of indices and a list of your strings and then zip them together:

ticks = [list(zip(range(5), ('a', 'b', 'c', 'd', 'e')))]

You can get an existing AxisItem of a PlotWidget like so:

pw = pg.PlotWidget()
xax = pw.getAxis('bottom')

And finally set the ticks of the axis like so:

xax.setTicks(ticks)

As far as I can tell, PlotWidgets automatically include 'bottom' and 'left' AxisItems, but you can create and add others if you desire.