Fixed the scatter part of fan

[paalge]

Plotting was not done on an axis, meaning that the plotting crashed, as Figure does not have the method scatter.

[ksterne]

Hello @paalge, a few notes here on this pull request:

• Have you had a chance to review our development structure? If not, it's in https://github.com/vtsuperdarn/davitpy/tree/master/docs/development_instructions . It's preferable that this type of pull request is made into develop instead of master as you've done here.
• In addition, can you describe more the problem and why this pull request should be merged in? This could/should include code you use to demonstrate the problem (makes it easy for someone to test) and a screenshot of the figure highlighting where the problem occurs.