Clear plots more aggressively, reset marker

Clear the plots to try to avoid the case where the bar chart item
has no X data, and reset the marker to avoid a bug in the vtkChartXY
class where it assumes a plot corner matching the one being used
means the plot was already added to the corner (not so when we
repeatedly clear the plot).
The previous call was not always disconnecting signals
Remove the scale bar from the view for now

It still needs some work.