Own dot graphics draft, multiline labels fix

[IvoryTowerDSPWizard]

Hi there! Thanks for the library :)
This PR is just an idea. I had to add some functionality.
Here each dot can have own draw function. It could also process mouseOver events there (I didn't add that). And it could have own predicate to define if a dot should be shown.

[IvoryTowerDSPWizard]

A couple of other things that could be improved.

• Not updating the plot canvas when no zoom/pan change occured
• Perhaps some data compression would be nice too - I added it in my client wrapper code to handle 700k+ data points. Each graph has several compression levels so that only a limited amount of objects is shown. Instead of N separate points you draw a rectangle with sizes that correspond to min and max values in that N range. That takes <2x the memory but exponentially increases the speed.
  
  

[IvoryTowerDSPWizard]

unfortunately I don't have the time to do a proper refactoring and PRs but I can provide my wrapper code so that you'd use the data compression logic that I've written

[jagracar]

Thank you for the PR! I'm quite busy at the moment, but I will check the code when I have some time and will integrate those parts that fit well. Thank you again!