Purpose
This displays the "altcoin index" as a plot on your chart. Suggested use is coupled with the "altcoin index oscillator".
Use
Add it as an indicator while looking at the chart you would like to assess. Click the settings button (cog) to select which coins you would like included in the index.
You can couple this tool with the "altcoin index oscillator" indicator to view the correlation.
Notes
The "altcoin index" is the average BTC price of a selection of altcoins. Therefore this is their relation to bitcoin and not any national currency. Due to this, the indicator will be useless if you try to use it with BTCUSD etc.