Closes #391: Pad series text to be same length as x axis

[washort]

Labels were not being displayed properly for multiple series when they didn't all have data points for every value on the x axis. This pads it out with nulls to line them up.

closes #391

[emtwo]

Thanks for the fix, @washort! This looks good to me. I'm going to add some details for context and regression testing to be used in the future:

STR:

1. Create a query with this text:

WITH sample AS
    (SELECT 942094 as client_count, 20180410 as submission_date, 'App 2.0.1 - Android 22' as version UNION
    SELECT 38761 as client_count, 20180409 as submission_date, 'App 2.1 - Android 25' as version UNION
    SELECT 347636 as client_count, 20180429 as submission_date, 'App 2.1 - Android 22' as version)
SELECT * FROM sample

2. Create a bar graph visualization where X Column is submission_date, Y Columns is client_count, and Group by is version.

Before the bugfix: the y value does not show on hover:

After the bugfix: the y value shows on hover:

[washort]

Thanks!