The x-axis is represented by a linear. On the Format Trendline Label pane that appears go to the Label Options tab. Go to Formula bar and type.
Suppose for example you wanted to plot the relationship between the Fahrenheit and Celsius temperature scales.
This technique could be wonderfully helpful if you get the data in a fixed format and you update charts by simply copy pasting the new data. Enter cell references with pointing. Enter a title by clicking on Chart Title. Enter descriptive headers into cells A1 and B1.