In this graph/chart at the bottom showing pace and heart rate, can you tell what is the pace and what is the heart rate? If the answer is 'no', then please add legends on the left and right axis accordingly to it please. Thank you.
If you tick off the heart rate you will see that the blue line for the pace remains and a legend on the right side appears. In general you are right of course, displaying lines without any indicators what they are for is a very basic design error.