I have created a graph to track money I have in certain accounts. I'd like to see the trend.

It's a bar chart, of assets and liabilities, showing in detail "Totals", for the last 12 months.

When I select all asset and liability accounts, a graph is shown.

If, however, I select (i.e., filter by) any of the accounts, then nothing is shown. But if I select all accounts except for that one, also nothing is shown. This completely clashes with my understanding of what an asset & liability graph should show: what is not shown in the first graph must appear in the second (because it's the "negation" or "complement").

What is wrong here?

I'm using KMyMoney Version 5.1.1-2b96a6d on Ubuntu 18.04 64bit.

I opened the default net worth graph report and the chart is shown. Modifying the account selection modifies the graph. Not sure what your problem could be