Garmin Data Analysis Webapp - feedback needed!

Hello Indie Hackers!

I've been working on a webapp to help users of Garmin fitness tracker devices to analyse their data. It's pretty rudimentary right now, but I thought it's time to get some feedback to try to guage market interest before writing any more javascript code ( current line count approx 2k! :O )

You can find the web app at:

http://www.amelyn.com/fitnalysis/fitnalysis.htm

As for the data, you can either export it off your Garmin device, or you can go to the Garmin webpage at:
https://developer.garmin.com/garmin-connect-api/sample-data/
and click on the link at the bottom of the page labelled "Mutiple Activity TCX" to download the sample data file.

Once you've got a data file, use the Browse button at the top of the web app to select your Garmin data file and upload it to the app. After a second or two, it will display five different graphs, each displaying a different analysis/breakdown of the first event in the uploaded data.

A dropdown box at the top of the screen allows the user to select one of multiple events in the data file, with each event representing a single training session.

The five graphs display the following information:

Graph #1:
Heart rate throughout the entire training session

Graph #2:
Total time spent at different heart rates throughout the training session

Graph #3:
Distance covered per each 10% of the training session

Graph #4:
Time taken to cover each 10% of distance throughout the training session

Graph #5:
Percentage of total time spent at each heart rate range

The buttons at the bottom of each graph allow the user to (1) expand the graph to a larger size or (2) export the data for that particular graph in CSV format. The CSV data can be copied and pasted into a text file. If the text file is then saved with a .csv filename suffix it can then be opened in Excel or another spreadsheet program for further custom analysis.

Any feedback is greatly appreciated. I'm thinking of adding more features, along with a payment option. If anybody has any ideas about what I should charge for this sort of thing or what additional features they would pay for if I develop the app further, please let me know. Like everybody else starting up a new project, I'm sort of swimming in the dark here!

Cheers!

Miktor