Add support for line/slope chart where the slope is the default view

[sophiamersmann]

Core problem

For Graphers that have multiple chart type tabs, like line/slope charts, it's currently not possible to make the slope chart the default view.

Proposed solution

The config field tab currently supports table, map and chart (where chart defaults to the first chart type). Add support for line and slope as well.

Alternatives

We could also allow to configure slope/line charts (not that slope is the first chart type) and then keep the tab config field as is.