Are you using OH1 or OH2? In OH1 the ON/OFF value of the switch is transformed to 0/1, so that it can be displayed on a chart. In OH2, this is not possible (at the moment). You can however define a number item and create a rule which sets the number item to 0/1, according to the switch state. And then display the number item in the chart.
With this rule, you can also set the value to 0/10 instead of 0/1.