Sets the type of normalization for this histogram trace. By default ('histnorm' set to '') the height of each bar displays the frequency of occurrence, i.e., the number of times this value was found in the corresponding bin. If set to 'percent', the height of each bar displays the percentage of total occurrences found within the corresponding bin. If set to 'probability', the height of each bar displays the probability that an event will fall into the corresponding bin. If set to 'density', the height of each bar is equal to the number of occurrences in a bin divided by the size of the bin interval such that summing the area of all bins will yield the total number of occurrences. If set to 'probability density', the height of each bar is equal to the number of probability that an event will fall into the corresponding bin divided by the size of the bin interval such that summing the area of all bins will yield 1. default: None