featurerequest: widget.cartesian - multiple line-plots in same axis

i think it would make a nice addition to be able to plot multiple lines in the same Cartesian.

what i think needs to be done for this:
- [ ] extract line drawing to sub-class
- the subclass could be an self contained bitmap or group (what makes more sense here?)
- [ ] change to `add point` function
- [ ] add `draw` or `update` function (what is the better naming here?)
- [ ] color option
- [ ] handle multiple Plot-Classes

please let me know what you think!