I have started (not live yet!) adding a little profile chart to the bottom of the activity map. The x-axis is distance and not time so this semi-fullfills a long standing feature request for that. Mousing over the chart will move the marker.
The marker distance, altitude and gradient (for 500m or something) will be displayed above it. I am probably going to indicate gradient with different colours on the altitude plot.
You can toggle it on/off and change the height in settings for the map.