Yes, we are comparing items that cannot be compared! A chart that results from this data doesn't live up to the comparison, it's almost like comparing apples and oranges. What you should note carefully is that the value range of temperature spans between 0 to 12 (approximately) whereas the range for rainfall is between 30 and 70. The temperature is depicted in degrees celsius and the rainfall is in millimeters. Figure 1 below, shows a chart that depicts this data from January to December. Our example data for this tutorial pertains to the average temperature and rainfall in London across the 12 calendar months of a year.
Sometime, one value axis for a chart is not enough! Of course that observation is only true if your data demands a second axis.
