To create this kind of chart, you cannot use pages->create chart. Instead you need to use the built-in analyzer for the item you want to create the chart for: Go to your item‘s page (e.g. on the „Properties“ page or through developer sidebar shift-alt-d, or Settings->Items), then click „Analyze“.
This opens a chart for this item only which you can modify to display the desired data and then save as a new chart page.
To display a monthly aggregate from an increasing counter (water meter or electric energy counter) open the drawer at the bottom of the screen, select tab „coords“ and select buttons „Fixed“ and „Aggregate“.
Change to tab „Series“ and on the right side select „Aggregation->Difference of firsts“.
You can change the appearance further using the other availabe options, but this is all that‘s necessary to show „per (day/month/…) view of a counter value. The save button at the top right creates a new chart page. Yes, it‘s confusing that the „create chart“ page in the settings menu has totally different options, but it‘s great to have all the options available somewhere. If all options were in one place it would probably be even more confusing.
Hope this helps, the screenshot some posts above shows almost everything, except how to get to the screen in the first place 