Index Value Plot

http://sparkleberrysprings.com/v-web/b2/index.php?m=200607

Index Value Plots take a base value, typically zero and compare the relevant data values against each other by using the base value as the standard. So you'll have frequencies below the base value as well as above. The base value is also typically the average value taken. With Index Value Plots, absolute values are not relevant because as stated earlier, values can dip below the base value. The map above is measuring the precipitation percentages in Sub-Saharan Africa among three particular regions over the course of 200 years with the last 200 years being an estimate. This map qualifies because one can clearly see the base value and then the recorded precipitation percentages which rise and fall. Here two variables are used as well time and annual rainfall anomaly. The reason it is pertinent is that researchers can use it when studying climate change and be able to prepare ahead of time.