'Significant difference' option is available under Advanced Settings.
It is set to 'Off' by default.
The legend referring to 'Significant Difference' can be found under the chart.
Sample total is the base set by default. This can be changed under the Advanced Settings 'Base' option.
These icons display the 'Significant Difference'. They will be visible only if you click on the chart and display the values so the icons can be displayed next to them.