Implement bubble chart - basically a curve where the symbol size and the color are taken from a column. See

• the "coloredpoints" example in veusz where one can choose datasets in the "Scale markers" and "Color markers" settings.
• http://reference.wolfram.com/language/ref/BubbleChart.html
• http://help.plot.ly/make-a-bubble-chart/
• http://www.originlab.com/doc/Origin-Help/Match-Modifiers-by

Much of the required code for plotting a curve with symbolds is already available in XYCurve. We need to decide whether we want to create a new type of a curve (or chart) or use already available XY-Curve and add new settings in the dock widget for symbol size and colors (like in veusz). For the symbols we'll need to allow different shapes and colors (already available) and gradients.