Character string of 'normalize', 'scale' or 'none'.
If 'scale', subtract the mean and divide by the standard deviation.
If 'normalize', i.e., max-min normalize, subtract the min and divide by the max.
If 'none', no transformation is applied.
More information on what transformation to choose can be acquired here:
https://cran.rstudio.com/package=heatmaply/vignettes/heatmaply.html#data-transformation-scaling-normalize-and-percentize