Formatting grouped horizontal axis of Pivot Chart

[tunaaa]

Hi, I've got time-based data and I'm using a PivotTable to smooth data collected daily by grouping by in 7 day intervals and taking an average. I've plotted the smoothed data, but the axis shows each of the 7-day periods instead of the year as I would like (see here). I have tried to apply custom number formatting, but this appears to be ineffective (see here). The actual spreadsheet is here in case it's of any help. I'm using Excel 2010.

Thanks.