[BUG]: multi-add for chart specification unselects the checks variables

[pascaleproulx]

Describe the bug
There is an irritating issue with the variable selection dropdown when specifying a chart in Simulate for example. If you try to multi-select, you have to click super slowly because if not it unchecks variables as you check more.

To Reproduce
Steps to reproduce the behavior:

1. Simulate any model with the default simulate settings (i.e. 100 samples)
2. Click on dropdown to select the variables to show in chart
3. Try to select multiple things => as you click more items, it unselects the previously checked ones