Really, it should be a histogram for each election, the data having no meaning in the intervals between elections. But joining up the data points appears to reveal a cyclic pattern of several election intervals duration.
Superimposing the percentage of the total vote which goes to parties other than the Conservatives or Labour seems to indicate the end in 2017 of what had been a very long-term trend. It was in the 1970s when the non-Labour, non-Tory share of the vote was last as low as in 2017. There were five general elections in the ten years from October 1964 to October 1974 - if there were a cycle, it was a short one. But it might be unwise to rule out a revival in Liberal Democrat and UKIP fortunes in the next few years.
This is the full link to Cowling's data: drive.google.com/file/d/0B5Ik-g