dropna in lineplot to get rid of no lines due to nans

[gidden]

Please confirm that this PR has done the following:

• Tests Added
• Documentation Added
• Description in RELEASE_NOTES.md Added

Adding to RELEASE_NOTES.md

Please add a single line in the release notes similar to the following:

- (#XX)[http://link-to-pr.com] Added feature which does something

[danielhuppmann]

@gidden, is this still relevant?

[gidden]

Yes this fixes the line plot when not all data have the same years

…

On Thu, Aug 9, 2018, 15:53 Daniel Huppmann ***@***.***> wrote: @gidden <https://github.com/gidden>, is this still relevant? — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#72 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ABVAEeDT3vsRuYgckweUEEZ7FuhHEaMVks5uPD7EgaJpZM4VK7GV> .

[danielhuppmann]

resolved merge conflict, waiting for CI...