Bitcoincharts doesn't have any documentation for their charts API, so what I ended up doing to backtest was the following:

Get the chart with the timeframe that I want to backtest (for example: 5 months, hourly.)
Click the "load raw data" link below the chart.
Select the first line of the data, hold the mouse button, then press the "End" key on the keyboard.
Drag the mouse to the end of the last line, then copy the data (Ctrl+C or right-click, then Copy)
Then, paste into a new text file and modify the data to suit your backtesting program (I use Gekko, so I wrote a simple PHP script that gets each candle, formats the date to a UNIX timestamp and then returns a CSV file that Gekko can read.)

I hope that helps.