In SlideMagic, you do not have to worry about picking the right scale for your data chart. The entire chart adjusts itself to the numbers you type in. See the example below:
To make sure that a consistent scale is used for your entire chart, you need to place all your data points in one shape, instead of using multiple shapes for example for each month.
P.S. I have added this monthly sales comparison chart to the SlideMagic slide library so you can easily use it in your own presentations as well. Search in the app for ‘sales’ and it will pop up.