1. Play with the two checkboxes at the bottom of the Mini-cart settings, they can help you emphasize the trend if the mini-charts don't have to be scaled, compared to each other.
2. If you'd like to add a static minimum, simply subtract the value from your mini-chart expression, like this:
Sum(Sales) - 1000000
or using a variable:
Sum(Sales) - $(vMyStaticMin)
In this case, you don't need to worry about the static Max - QlikView will automatically scale the highest value in the chart to take up the available space.
3. If you'd like your Min value to be dynamic, you can try to calculate it using advanced aggregation. For example, if your chart is by Customer, then this will be the min value of sales by Customer:
Now, you can subtract this value from the chart expression. This way, the min. value of Sales will be equivalent to 0 in your mini chart. You can tweak it to show an arbitrary small amount in this case.