vtkChartXY: check the scene before use

This can happen when a chart is manipulated without a view.

Change-Id: Ifbe098beab74682511fcbb41f886349ebda0fffd
vtkChartXYZ: use dummy bounds rather for bounds calculation

If the first plot is empty, getting the point is a segfault.

Change-Id: I81d93b42b2ae27180b63d2f879baddf4d4e25b7b
vtkChartXYZ: don't copy the points out

Change-Id: Icc02bde790e152f52878692d74ee0afae027b0f4
Merge topic 'chart-segfaults' into master

93dac76f vtkChartXYZ: don't copy the points out
81eb3dea vtkChartXYZ: use dummy bounds rather for bounds calculation
458c079b vtkChartXY: check the scene before use