1. Create a graph widget and add two data sets.
2. Add existing item to data set 1 and select left Y axis.
3. Add existing item to data set 2 and select right Y axis.
4. In Axis tab, select to show both axis and set custom units like so:
left axis
Min: 9999
Max: 10000
Units: AAAAAAAAAAAAAAA

right axis
Min: 9999
Max: 10000
Units: BBBBBBBBBBBBBBB
5. See that preview is working. If no data is visible, custom units may not appear, but that's fine. Later in view more try to select a portion where data exists so that custom units are visible.
6. Apply changes then resize graph widget to minimum possible size.
7. Save graph widget and exit edit mode.
8. If custom units are not visible, select a time on graph where both items have data. So that custom units appear.
9. As a result the grid, axis and everything becomes so small due to custom units taking up large amount of space, and script gets stuck in some endless loop. So widget no longer loads and the error can be observed in the response:
Fatal error: Out of memory (allocated 179306496) (tried to allocate 36 bytes) in C:\Development\zabbix-4.0\frontends\php\include\classes\graph\CSvgGraph.php on line 382