jmcnamara/XlsxWriter — pull request #17 by abirmingham (2013-05-24 15:59:31+00:00)
Title: Added type 'custom' to error_bars on charts

Hey John -

For my project I needed Excel's 'custom' type for chart error bars. I've included a minor change to accomodate that.

Example usage is as follows:
```
chart.add_series({
'categories': '=A2:A8'),
'values': '=C2:C8',
'y_error_bars': {
'type': 'custom',
'value': ('=B2:B8', '=D2:D8'),
'direction': 'both'}})
```
In this case my cell composition looks like this:

| | First Quartile | Median | Third Quartile |
| ----------------- |:-------------:|:-------------:|:-------------:|
| Red | 12 | 15 | 25 |
| Blue | 10 | 45 | 60 |
| etc | | | |

Please let me know what changes/tests you would like in order to be merge-ready. Thanks!