Microsoft Excel has readily usable charting/graphing capabilities. Previous versions of Excel had a wizard to guide you through creating a pie chart. More recent versions of Excel allow you to highlight the data to include in the chart/graph and press a few buttons to create it. The pie chart will show how each part of the data contributes to the whole.
Enter the months of January through December in column A, for example. In column B, enter the electricity bills for each month. The pie chart will show each month's percentage of the year's electricity bill. This data can be actual or fictitious.
Click and drag over the data to highlight it. The data that's blue will be the data included in the pie chart.
Click on the "Insert" tab and then click on the down arrow on the "Charts" panel.
Press the down arrow under the "Pie" option. Click the first option under "2-D Pie." The pie chart shows beside the data.
Right-click in the chart area to format the appearance of the pie chart. In this mode, you can add a title to the chart and change the fonts. To display percentages instead of actual values, right-click on the data then click "Format Data Labels." Under "Label Options," place a check beside the "Percentage" option. Uncheck the "Values" option. Click "Close." Grab the borders of the pie chart to resize it and move it around the page.
Any change in the data will cause the pie chart to change.