Carl Klarner, of Indiana State University, has put together a very interesting data set of the history of partisan composition of state legislatures. This page visualizes the information for upper chambers (state Senates).
Adjust the slider below to change the start or end year (states will sort by change in Democratic percentage over the period, with a bit of a bug where states have missing years).
Click any state's chart to see a larger version that also includes Presidential numbers. Hover over the circles on the larger version to see the specific numbers. States before the first vertical line show an increase in Democratic seat share over the period, and states after the second vertical line show a decrease.
*On Dates: For a given year, Klarner might provide two partisan counts--one immediately after an election, and one for the session itself. I decided to use the post-election numbers where they were provided, and the session numbers otherwise.
Depending on the timing of special elections, vacancies, party-switches, and other changes, it is possible that any differences between these two numbers happened after the start of the second year, or that some change at the start of the second year was reversed by the time of the session numbers, an so on. There are a number of discrepancies in the overlapping years between Klarner and the NCSL, even using the approach that I used, with many of these apparently caused by Southern legislators who switched parties after the 2010 elections but before the NCSL snapshot. Klarner has a very detailed spreadsheet of sources and explanations of how he reconciled different numbers from his own sources at the above link.
On Sources: Klarner's spreadsheet, as well as the more recent NCSL numbers and Stephen Wolf's presidential numbers, are linked above. I used Tabula to extract tables from the NCSL PDFs.
The brush slider is adapted from Mike Bostock's brush snapping example, which was Sam Petulla's idea. The use of "small multiples" was inspired by Lena Groeger, and the URL stuff remains adapted from Mike Bostock's "Paint By Numbers" example.