Plotting stops after 10 hours of measurement, measurement continues

### Steps to reproduce
1. Create a measurement of multiple parameters and dimensions
2. Run the measurement for a long time i.e. 10 hours
3. Plot the data in ´Qc.QtPlot´ (I only plotted the z-values, no x,y)
### Expected behaviour

The Data should be plotted while it is beeing measured
### Actual behaviour

The plotting stops after 10h, while the measurement continues.
The notebook is unresponsive.
### System

**operating system**
Windows 7

**qcodes branch**
Merge of Master, Merlins instruments, array id names, file location formatter

@alexcjohnson @giulioungaretti

Start by reproducing a long-running multi-parameter measurement on Windows 7 and plotting z-values through Qc.QtPlot, as described in the issue. Investigate where plotting becomes unresponsive while measurement continues; done means the plot continues updating and the notebook remains responsive beyond 10 hours.

Written by the indexing model from the issue text.