To my knowledge what you want isn't possible in that way. The custom fields are stored with either athlete or ativities.
But something like this should give a value: (currently i don't have a pc with ST in front of me)
{somefield(Sum,7)} sums the value of somefield in the last 7 days, counting from the day of the activity on which the formula is executed. I'm not sure wether it is zero based or 1 based So today will be 1 or 0. My guess is 1. And the value for today - 1 is the difference of (Sum,2) - (Sum,1) So counting to 7 makes it (Sum,8) - (Sum,7)
The formula will be something like: {somefield(Sum,1)} - ({somefield(Sum,8)} - {somefield(Sum,7)})
However some thoughts.
- CTL isn't a linear function.
- The way your graph works depends on the way the summarize option of the specific customfield is set, check this
- Both the training load plugin and the fellrnr plugin provide graphs to show your freshness or training status, so why graphing this if there are allready graphs