Fixed assertion, when a plot area with gradient background is painted with zero width or height.

Change-Id: Ic797b83d62ba7ab61e5855d9618603ddcb883e38
Merge "Fixed assertion, when a plot area with gradient background is painted with zero width or height."
Fixed annoying Overflow exception when zooming into plots.

These were caused, when the physical coordinate of any plot point exceeded +/- Int32.MaxValue (e.g. due to a strong zoom on a large span of data). Also improved performance of painting code,

Change-Id: I75b49dbe6935baa330aab333f343474c61a73dae
Merge "Fixed annoying Overflow exception when zooming into plots."