I use Watt2kwh. I store its metrics in a variable, "energyVal" until
at 24:00 when everything is reset and starts over.
How can I present the "energyVal" variable in a bar chart for each day for 1 month.
thanks for the help 
Define a month?
Where do you wish to display, a dashboard? which one?
Yes a month is a month. 30, 31 days works fine. It should be built up by the variable and present every day. then it can be set to zero.
Dashboard, I don't know really and can put it where it fits, big large what ever!
Do you need to display today's energyValue only at midnight or must the bar reflect consumption through the day?
You probably need to store each day's total as an array in persistent context.
At midnight, add the new data then if the array has a full month's worth of data, drop the first element.
Feed this to the chart node as stored data.
You are being very vague with information.
Here are two examples
1st will add a value every day and reset stored data on the !st day of the month.
2nd will maintain a store of last 31 days.
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Using node-red-dashboard (node) - Node-RED
Hope this helps you get started.
No it's not need for presentation during day. I only need to show the day before.
Thanks I'm going to take a look on your examples
Yes an Array could solve the wishes. But I'm bad in coding.
Thank you so much
