Assume all the incomes will be spending on consumption.
Consumption (Heat) = Income-Consumption (others)
Now income increases sine the subsidy on heat and consumption on others remains.
That is the ability of consumption of heat increases.
If the consumer remains to use gas,his consumption ability is remained at the same level.But if shift to other alternative,he’s benefited from subsidy.
X-axis is Income and Y-axis is Consumption