Create an Number item. Link it to http binding in conjunction with jsonPath transform. Then use rrd4j persistence (simple) or InfluxDB/Grafana (more complex, better look) to create your chart.
Define an two additional number items. Dont link it to any channel. One serves as memory for the last sensor value, one as the daily counter.
Setup a rule with a cron trigger. In that rule you have to:
Determine the date change and set the daily counter to item to zero.
Setup a rule triggered by a change of the item linked to your sensor. In that rule you have to:
Add the difference between the last sensor value (memory item) and the current sensor value to your daily counter, then store the current sensor value in the memory item.
In obe of the Homematic forums there is a script to transform the regualr reading into different value. It should be not to complicated to move that to openHAB.
I read out my total power usage and save it to InfluxDB and then get the daily usage by executing a query to the InfluxDB. This way I only persist one number and can get all information based on that.
I query the DB with a bash script using a midnight epoch time variable.
midnight=$(date -d "$today 0" +%s%3N)
SELECT max("value") - min("value") FROM "MeterDevice_Household_CounterTotal" WHERE time > '"$midnight"'ms'