When choosing a Line chart in Excel you will always get a vertical drop off after the last entry, if the "next month" is 0, (zero), or has not yet been entered. It is a nuisance and the only way I can think of changing this is to limit the x axis, (months), so that the last month is always the last for which you have data.
But that means you have to change the x axis range each time new data is entered.
My own option is to use a different chart type. I use the Area chart, 1st sub-type. This creates an area below the line which ends with the last data entry.
Not always acceptable, but worth a try to see if it works for you.
I have a table that has months across as columns and products in column a. for each product (and month) there is a number associated with it. for months jan - apr, I have values in those cells, but not in may - dec. when I create a line chart using the whole range (jan-dec) as my source, my line drops off to zero at may, when in reality the value is not zero and I just want the line to end at april. any suggestions ?
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