This sales-by-month-template.pptx example has a line chart with two data points. The shell command below uses setChartData to add values for eight more months, then downloads the result as sales-by-month.pptx in your current directory.
The X axis is set up in PowerPoint as a date axis, which stores dates internally as the number of days since Jan 1, 1900 and allows them to be formatted independently, in this case showing just the month. We specify the dates to setChartData in YYYY-MM-DD format, from which Slide Tailor converts them into numerical days. Alternatively we could work out the numbers 45292, 45323, 45352, ... beginning those months and specify them as JSON numbers. Had we configured the X axis to treat "Jan", "Feb", "Mar" as strings, we would have to specify them as strings in the JSON.