What is the unit of Y-Axis in drought severity graph?

Drought Severity Unit

Y-Axis is severity and doesn't have unit. Its unit depends on your index (CZI). The tallest column is 21.5 that calculates (in Rain based Drought Indices Tool): Severity= abs(CZI_1963.11-border) + abs(CZI_1963.12-border) + abs(CZI_1964.1-border) + .......+ abs(CZI_1965.9-border) = 21.5

Monitoring is normally performed using drought indices. Drought indices provide decision makers with information on drought severity and can be used to trigger drought contingency plans, if they are available. As you know, the range of drought indices and their threshold are not similar. For better clariffications, it is better to refere the origin paper of each droght index. However, In this sample, I think you selected threshold = 0. It's not good. Please read Mckee 1996. You should select threshold= -0.99 you can visit SPI threshold