When using markers, you have to set a range to determine where the marker will be placed. The range by default takes indexes but if you are looking to place the marker by value you need to set valueRange:true. This will allow the range to take values instead of indexes.
I have created a demo to show how this is done:
This is a demo showing how range without valueRange:true is done.
Here is a demo showing how range with valueRange:true is done.