There have been 18 by-elections so far in this parliament – more than any parliament since 1992-97.

The Liberal Democrats have contested 16 of the 18, and have won only one: Eastleigh, which the party held with a reduced majority. They haven’t gained a single extra seat, and have lost their deposit nine times.

Here’s a graph of the full horror story. In chronological order, I’ve listed the change in percentage points between the Lib Dems’ vote share at the 2010 general election and in the subsequent by-election:

You can see that the party’s average drop in vote share has got worse as the parliament has gone on.

In the top half of the list, the drop is above 10 percentage points only three times.

In the bottom half, the drop is above 10 points every single time except one (and even then it’s 10 points exactly.)

The decline of the Lib Dems as a by-election fighting force is one of the most persuasive stories of this parliament. But it’s a trend that stretches back beyond 2010.

The last time the Lib Dems gained a seat at a by-election was in February 2006, when it took Dunfermline and West Fife from Labour. This was also the first time Labour had lost a seat at a parliamentary by-election in Scotland since the SNP won Glasgow Govan in 1988. Labour retook the seat at the 2010 general election.

That was the only occasion the Lib Dems gained a seat at a by-election during the 2005-2010 parliament. But they managed it twice during the 2001-2005 parliament, in Leicester South and Brent East. Both were gains from Labour.

They did it just once in the 1997-2001 parliament, when they took Romsey from the Conservatives.

The peak of the Lib Dems’ by-election insurgency was during the 1992-97 parliament, when they gained four seats, all from the Tories: Littleborough and Saddleworth, Eastleigh, Christchurch and Newbury.

They also picked up three seats at by-elections during the 1987-92 parliament, in Kincardine and Deeside, Ribble Valley and Eastbourne: again, all from the Tories.

That’s as far back as we can go, because that’s as far back as the Liberal Democrats go.

We know where the story began – but can we say for sure the story is now over?

It would be tantalising to have a by-election in a marginal seat where the Lib Dems are in second place – Camborne and Redruth, for example, where sitting Tory MP George Eustice has a majority of just 66. Sheffield Central would be even more fascinating, where Labour has a majority of only 165.

How the Lib Dems performed in these kinds of contests would really help to tell us if the party’s days of by-election victories were over. Until then, we can only contemplate an ever-swelling list of diminishing returns and disappearing deposits.