Why You Can't See Speed Graph on Run Activity

Not criticizing your request but Pace and Speed are just basically inverse of each other (you only need one to calculate the other).  Although there are reasons to use speed for training there tends to be more reasons that pace is a better/easier metric (yes I know - not always true).  From a race perspective, most are fixed distances so dealing with the time it will take to do a km (or mile) is more relevant than how far you can go in an hour -  a few seconds per mile (or km) makes a lot of difference while a difference in speed between 6.0mph and 6.3mph means much less to most of us.  In short, Pace vs HR is really the same thing as Speed vs HR - just inverse - with Pace, the higher the graph the slower you are going.  Lots of articles available explaining this - and in the end this is the way most do it.