Default yAxis title should not be "Valeur"

Default yAxis title should not be "Valeur".

The default value must be empty with an option to override it.

``` javascript
text: typeof $table.data('graph-yaxis-'+yAxisNum+'-title-text') != 'undefined'  ? $table.data('graph-yaxis-'+yAxisNum+'-title-text') : "Valeur"

```
