Custom position for the legend

[lorentey]

• The chart should be improved so that the legend is automatically placed in a corner where it doesn't intersect any of the curves.

• I should also be able to click and drag the legend to move it somewhere else, in case it happens to cover some interesting area.

• There should be a command to forget the custom position and restore automatic layout.

The chart renderer already includes support for drawing the legend at a custom position. The position is given as a relative offset vector to one of the corners of the chart. It is important to anchor the legend to one of the corners, so that the user does not need to keep repositioning the legend after resizing the chart.