Two Y-scales in pyqtgraph (twinx-like)

Question

How can I generate a plot with two Y-scales in pyqtgraph?

I also need the two in different colors (corresponding to lines' colors).

In matplotlib it can be done using twinx, as in this example.

If there's no way to do it with a single plot object, perhaps there's a way to overlay a plot (with y-axis on right side) on another one (with the y-axis on left)?

Answer 1

See pyqtgraph/examples/MultiplePlotAxes.py.

The solution is just what you described--overlay two PlotItems.

Answer 2

Here is some code that I think shows a practical example of what it is you are after. This is an expansion of two pyqtgraph examples: PlotSpeedTest.py and MultiplePlotAxes.py.

from pyqtgraph.Qt import QtGui, QtCore
import numpy as np
import pyqtgraph as pg
pg.setConfigOptions(antialias=True)
pg.setConfigOption('background', '#c7c7c7')
pg.setConfigOption('foreground', '#000000')
from pyqtgraph.ptime import time
app = QtGui.QApplication([])

p = pg.plot()
p.setXRange(0,10)
p.setYRange(-10,10)
p.setWindowTitle('Current-Voltage')
p.setLabel('bottom', 'Bias', units='V', **{'font-size':'20pt'})
p.getAxis('bottom').setPen(pg.mkPen(color='#000000', width=3))
p.setLabel('left', 'Current', units='A',
            color='#c4380d', **{'font-size':'20pt'})
p.getAxis('left').setPen(pg.mkPen(color='#c4380d', width=3))
curve = p.plot(x=[], y=[], pen=pg.mkPen(color='#c4380d'))
p.showAxis('right')
p.setLabel('right', 'Dynamic Resistance', units="<font>&Omega;</font>",
            color='#025b94', **{'font-size':'20pt'})
p.getAxis('right').setPen(pg.mkPen(color='#025b94', width=3))

p2 = pg.ViewBox()
p.scene().addItem(p2)
p.getAxis('right').linkToView(p2)
p2.setXLink(p)
p2.setYRange(-10,10)

curve2 = pg.PlotCurveItem(pen=pg.mkPen(color='#025b94', width=1))
p2.addItem(curve2)

def updateViews():
    global p2
    p2.setGeometry(p.getViewBox().sceneBoundingRect())
    p2.linkedViewChanged(p.getViewBox(), p2.XAxis)

updateViews()
p.getViewBox().sigResized.connect(updateViews)

x = np.arange(0, 10.01,0.01)
data = 5+np.sin(30*x)
data2 = -5+np.cos(30*x)
ptr = 0
lastTime = time()
fps = None

def update():
    global p, x, curve, data, curve2, data2, ptr, lastTime, fps
    if ptr < len(x):
        curve.setData(x=x[:ptr], y=data[:ptr])
        curve2.setData(x=x[:ptr], y=data2[:ptr])
        ptr += 1
        now = time()
        dt = now - lastTime
        lastTime = now
        if fps is None:
            fps = 1.0/dt
        else:
            s = np.clip(dt*3., 0, 1)
            fps = fps * (1-s) + (1.0/dt) * s
        p.setTitle('%0.2f fps' % fps)
    else:
        ptr = 0
    app.processEvents()  ## force complete redraw for every plot.  Try commenting out to see if a different in speed occurs.
timer = QtCore.QTimer()
timer.timeout.connect(update)
timer.start(0)


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