I would like to have the option to see different periods of time combined on my personal heatmap. As it works now, I can see all of my activities combined, or I can look at any individual year. What I would like is the ability to toggle years on and off, and to see any combination of years together. For example, it would be nice to see all of my 2015 activities, then to layer 2016 on that, and 2017 on that to see my progression over time. This should be easy to accomplish since you already have the ability to filter activities by year.