[Lazarus] TAChart with transparent background

[Werner Pamler]

I see. If you can keep the chart size unchanged then it is certainly a 
good idea to stretch the image to the requested size immediately after 
loading and use a simple "Draw" of the obtained image while charting. If 
you know the range of your data in advance it may also help to freeze 
the chart extent to avoid unnecessary repaints when data arrive. And you 
could also enclose the AddXY calls by calls to Begin/EndUpdate of the 
ChartSource. Avoiding unnecessary repainting can have a dramatic effect 
on speed.