I've been using this app for a while now. While I really like the idea I haven't included it into the list of my favorite useful apps, because there are some points that I find confusing:

1. The images of the diagrams on top suggest that clicking on them opens a corresponding diagram. That's not the case and thus counter-intuitive. It would be better to make four buttons: Intervals, Tracks, Invteral Diagram and Tracks Diagram.
2. The Interval Diagram should be a global diagram that displays all interval tracks above each other in different color (which the user can select by long pressing on the specific activity). (I haven't used tracks, so I won't comment on them.)
3. Pressing on the menu button on my Android phone leads to no response. At least a help file should be implemented.
4. The pinch zoom function in the diagram screen is counter-intuitive. Usually you pinch to zoom out. There's a reason for that convention: When pinching on a map the distance of the points on the map on which your fingers are placed on remains constant while pinching. Those points on the map remain "glued" to your fingers, which is pretty intuitive.

Anyway, I really appreciate your efforts at creating this app! :)

If you ever decide to make a paid version, a price between 0.99$ and 1.99$ would be appropriate for such a simple app. You get superb Android apps (for which big teams of programmers worked for months) for as little prices as 4.99$.

Further idea: A feature for computing correlations between the different activities would be great. What might make most sense is to take the seconds, minutes or hours per day or week an activity is happening as discrete random variable. For example: X: {days from the beginning of any activity until now} -> {0,...,1440} X(d) = minutes of reading fiction on that day

Having separate graphs for those variables would be pretty useful, too.