Qcodes: PyQtgraph live plot does not update

Issue made to get rid of the hack in PR #250 .

Steps to reproduce

1. Run Tutorial notebook from #250

data = qc.Loop(c0.sweep(-20,20,0.1), delay=0.003).each(meter.amplitude).run(name='testsweep')
plotQ = qc.QtPlot()
plotQ.add(data.meter_amplitude)

Expected behaviour

Expect to see a live updating plot of the data

Actual behaviour

Plots only what is in “data” the moment the plot function is called.

Workaround

while data.sync():
    plotQ.update()
plotQ.update()

The following code will keep updating the plot (in the main thread) while the data taking is not finished yet.

System

operating system OS X El Capitan 10.11.5 (15F34) qcodes branch “examples” (only contains notebook so should be close to master) qcodes commit Commit: 007e5f95ac5