Sometimes your data type may require a second axis. The sample data that we took for this tutorial pertains to the average temperature and rainfall in London across the 12 calendar months of a year. The temperature is depicted in Celsius and the rainfall is in millimeters. What you should note carefully is that the value range of temperature spans between 30 and 70, whereas the range for rainfall is in between 0 to 12 (approximately). A chart that results from this data doesn’t live up to the comparison since we are trying to compare items that are from different data ranges.