Fixed bar chart bars moving on color update.

Fixed an issue where bar Charts would occasionally move when updating their colors. As you can see from the images, it looks like it's recalculating the distances between the datums incorrectly.

> This fix is almost certainly not the 'correct' way of fixing it, but it'll hopefully get you started on a real fix.

### Bug ###
**Pre Color Change:**
![screen shot 2014-06-06 at 11 54 22 am](https://cloud.githubusercontent.com/assets/130903/3204648/0b9284a4-edac-11e3-9f9c-dfb544ff9214.png)

**Post Color Change:**
![screen shot 2014-06-06 at 11 54 37 am](https://cloud.githubusercontent.com/assets/130903/3204649/104fb12e-edac-11e3-990b-e124d38ed132.png)

### Reproducing ###
Code: [Link to Commit affected](https://github.com/scoophealth/scoop-dashboard/tree/c1be8e7d621b0d46101cdf2b51c68659751a0457)

You can just `npm install` in the folder and run `grunt serve` to host it, no external dependencies.

Open your browser to `localhost:8080/#/query/4`, login with `foo`/`bar` as details, and change the colors via the selectors.