You need gpxpy. Install it via
gpxpy
$ pip install gpxpy --user
Run python elevation_chart_gen.py -f ../Downloads/Jul_3,_2016_12_52_51.gpx to generate the data.csv. Then run make to generate the chart.
python elevation_chart_gen.py -f ../Downloads/Jul_3,_2016_12_52_51.gpx
data.csv
make