This is not possible. It is a Processbook behavior. The method for assignment is:
Since string tags do not have an inherent numerical value, ProcessBook assigns an integer value to each different string it encounters. The first string it encounters gets 0, the next gets 1, etc. Every time a display is opened that contains a trend with a string tag the string tag can be plotted differently; depending on the values returned for the string tag when the display is opened - again the first string encountered during the plot time would be assigned 0, the next would get 1, etc. Different string tags are assigned numbers in the same way. A different string tag, even with the same string values, could get different number values if that string did not arrive in the same order when it was trended.
That's what I suspected, the vaues being created "client-side", after the SDK call. I have recreated this logic on a web service, which works great, but after the chart is populated and starting to receive realtime data I have no way of assigning the correct integer values unless I fetch the entire time range everytime or sends those data back from the client to the web service.
I will see if I can do it client side for realtime data.
Thank you for your answer.