When there's a single DataSet selected, the chart shows real values on the value axis. Now, when you select at least one another DataSet for comparison, the chart goes into "percent" mode redrawing all the graphs and value axes to show percent change from datapoint to datapoint rather than real values.
It's a standard practice in Stock chart. However, it's not always applicable for your needs.
To disable such behavior, and make the chart always show real values use the following ValueAxesSettings property:
chart.panelsSettings.recalculateToPercents = "never";
You can also set this property per panel also:
stockPanel.recalculateToPercents = "never";
Alternatively, if you like to enable recalculation to percents even if there is just single DataSet, you can use always as the above setting:
chart.panelsSettings.recalculateToPercents = "always";