Lines are not connected when data granularity is less than 1 month

[colincadams]

Describe the bug

If there is a month with missing data, or data is reported quarterly or annually, then there are no lines on the graph.

To Reproduce Steps to reproduce the behavior:

1. Replace src/data.json with https://drive.google.com/file/d/1OLenZP9LW_p3SKX1kA7JahL7n6R-nuxk/view?usp=sharing
2. Change stateCode to US_CT in src/index.js
3. Run yarn start
4. Open the site and scroll to population graph
5. Change time to "5 years"
6. Notice the probation population graph is invisible, but only appears when hovering over points

Expected behavior

@hobuobi to confirm: I believe there should there be a line connecting these (alternative is just make the points visible with no line)

Screenshots Screenshots revealing sensitive information must not be shared. If a screenshot is helpful in diagnosing and resolving the bug, please send this directly to a contact on the Recidiviz team.

Desktop (please complete the following information):

• OS [e.g. Windows 10]: MacOS
• Browser [e.g. chrome, edge]: Chrome
• Version [e.g. 22]:

Additional context Add any other context about the problem here.