The Eval Graph displays the scores present in the original PGN file, but in analysis mode these are never altered. The stored scores are normally only altered when an engine makes a move, and in analysis mode it never does.
Or are you talking about the 'Analyze Game' function?
But the engine would not have seen most of the positions in analysis mode. Only the one you are currently analyzing. You want the score of that one position to be automatically updated? I am not sure that would be helpful. When analyzing variations you might visit the same position several times, and each time the engine would start analyzing it from scratch. So you might overwrite scores from a long analysis with scores from a much shorter one.