A simple HDMI to via cord isn't going to work. HDMI is digital and via US analog. You need a HDMI to via converter. There is some good info on this forum post.
Here is the pyhton code that reads the serial data from the Arduino.
I'm no coder just a hack so please forgive my code form as this isn't cleaned up. Some of the includes are not necessary but I copied this from another project I have and was to lazy to remove the un-needed ones.
Let me know...