Raster plot is not cleared if a new simulation causes no spikes

[sebjameswml]

To reproduce, make a network which produces a spike raster and plot it in the graphing interface. Now change a parameter to prevent any spiking and re-run the experiment. The plot still shows the spikes from the last run. You can verify that there are no spikes by "clearing all graphs" from the plot and then re-plotting the raster.