On the Data tab, towards the bottom there is a "+ Sort" button where you can select a field to sort the serial chart. If you used Fields to create the serial chart then this option isn't available.
If you turn off the Parse Dates functionality, then Sort By option becomes available. There doesn't appear to be a way to sort categories with the Parse Dates functionality turn on.
When I do that, I can sort by date, but I want to sort by analyte name in the legend. Currently, they don't sort:
It appears as though this option is not available.