Histograms display the frequency distribution of data of a single variable. In ArcGIS Pro, the symbology histogram visualizes the distribution of data across the chosen symbolized field. You can use this histogram to view whether the data follows a symmetric, skewed, or modal pattern, and how the data can be symbolized to show outlying or prominent features on your map.
You can analyze and edit the histogram on the Histogram tab in the Symbology pane. Histograms can be viewed and edited with these types of symbology:
- Graduated colors
- Graduated symbols
- Bivariate colors
- Unclassed colors
- Proportional symbols
- Symbology varied by color
- Symbology varied by transparency
To create and export a stand-alone histogram for extended analysis or reports, consider making a chart.
View the histogram of your symbology
To analyze the symbology histogram for your feature layer, complete the following steps:
- In the Contents pane, click a feature layer to select it.
- Under Feature Layer, on the Appearance tab, in the Drawing group, click the Symbology button
to open the Symbology pane. - In the Symbology pane, on the Primary symbology tab , ensure that one of the primary symbology types listed above is selected. Set the field, color scheme, and other symbology properties as needed.
- If necessary, click the Histogram tab to view the distribution of the data in the field in the context of the current symbology. For bivariate color symbology, click Field 1 Histogram or Field 2 Histogram.
The format of the histogram is based on the primary symbology, its color scheme, and whether the data is classed. By default, the minimum and maximum values of the symbolized field are displayed at the top and bottom of the histogram. The color scheme also shows transparency if applied directly to the symbol. The data is divided into horizontal columns that represent a frequency count of the data in that percentile.
Tip:
Hover over the average (mean) marker in the histogram to view the average value of the data. The value is dependent on the sample size, which is set to 10,000 features by default. You can change the sample size in Advanced symbology options 
.
Modify symbology with the histogram
You can use the histogram to directly change how values are symbolized in your feature layer. For example, if you are symbolizing a map of business locations and you want to emphasize locations with a customer satisfaction rating below 80 percent, you can set the minimum value in the histogram to this value so that all locations with a rating under 80 percent are drawn with the same symbol.
From the histogram's tab on the Primary symbology tab , in the Symbology pane , you can do the following:
- To change the value, double-click the minimum and maximum values, type a number, and press Enter. If a value is outside the range of the maximum or maximum value in the histogram, it does not draw on the map, and the histogram reflects this by stretching the height of the horizontal columns.
- To edit the value, double-click or right-click any value stop and click Edit value. You can also drag slider controls up or down to manually set the value. The value must stay within the range of its neighboring break values.
- To view statistics for the symbolized field, click the More drop-down menu and click Show statistics. This table displays the count, minimum, maximum, mean, and standard deviation of the field, which you can use to further define class breaks or value stops.
- To view the distribution of the data more easily, drag the expander bar above the histogram upward to make it larger in the pane.
Depending on the symbology of your feature layer, the histogram has additional tools, as described below. For example, with some classed data, you can manually add class breaks and set class ranges.
Graduated colors
The chosen color scheme and number of symbol classes are displayed in the histogram. The classification method determines the size of each class interval, and each class interval (the uppermost value of the class) is represented with a label and an adjustable value stop.
Note:
Any custom edits made to class break intervals automatically change the classification method to Manual Interval.
- To add a class break, right-click anywhere on the ramp and click Insert break. To remove it, right-click the break value and click Remove break. Legends in the Contents pane and layouts update automatically, and class labels persist if previously edited.
- To emphasize an important class break value in a feature layer, right-click the value and choose Set as critical break. Then, click the Color scheme drop-down menu and choose a diverging color scheme. To remove the critical break, right-click the class break value and choose Remove critical break.
- To reverse the color scheme, click the More drop-down menu and click Reverse symbol order. To reverse the values but maintain the order of the color symbols, click Reverse values.
- To reset or update the symbology, click the More drop-down menu and click Regenerate all symbols.
Bivariate colors
Bivariate color symbology has two histograms, one for each symbolized field. Similar to other quantitative symbology types, the classification method determines the size of each class interval. However, the number of intervals is controlled by the grid size. You cannot manually add or remove class breaks in the histogram.
Note:
Any custom edits made to class break intervals automatically changes the classification method to Manual Interval.
- If the source data has been changed or updated, click the More drop-down menu and choose Refresh values. Refreshing values maintains the color scheme and direction of values but undoes all other previous edits to the histogram.
- To reset or update the symbology, click the More drop-down menu and click Regenerate all symbols.
Graduated symbols
The ramp of a graduated symbols histogram represents the relative size of the graduated symbol in each class. If the symbol template is not a shape marker symbol, the color of the symbol may differ from the color scheme in the histogram.
Note:
Any custom edits made to class break intervals automatically change the classification method to Manual Interval.
- To add a class break right-click anywhere on the ramp and click Insert break. To remove it, right-click the break value and click Remove break. The Contents pane and layout legends update automatically, and class labels persist if previously edited.
- To reverse the size of the symbols in your feature layer, click the More drop-down menu and click Reverse symbol order. To reverse the values but maintain the order of the symbol sizes, click Reverse values.
- To update the symbology based on newly added breaks, click the More drop-down menu and click Regenerate all symbols.
Unclassed colors
Because the data is not classed, there are no class breaks in the histogram. It displays the distribution of values between the minimum and maximum value.
- To adjust the distribution of the color scheme, drag the upper or lower slider control up or down. Any data with a value greater than the maximum slider value draws with the color shown above it. Any data with a value lower than the minimum slider value draws with the color shown below it. Any variation in the color scheme is applied to the data between these two controls.
- To reverse the color scheme, click the More drop-down menu and click Reverse color scheme. To reverse the values but maintain the color scheme, click Reverse values.
- If the source data has been changed or updated, click the More drop-down menu and choose Refresh values. Refreshing values maintains the color scheme and direction of values but undoes all other previous edits to the histogram.
Proportional symbols
The format of the histogram is a representation of the relative size of the proportional symbol applied. If the symbol template is not a shape marker symbol, the color of the symbol may differ from the color scheme in the histogram.
- To adjust the distribution of the proportional symbols, drag the upper or lower slider control up or down. Any data with a value greater than the maximum slider value draws with the maximum size symbol. Any data with a value lower than the minimum slider value draws with the minimum size symbol.
- If the source data has been changed or updated, click the More drop-down menu and click Refresh values. Refreshing values resets the symbology and undoes all previous edits to the histogram.
Note:
The proportional symbols histogram cannot be edited if units for the data are set.
Variable attribute histograms
If you choose to symbolize a field using additional attributes such as transparency or color, an editable histogram displays below the attribute on the Vary symbology by attribute tab 
in the Symbology pane . This histogram is a visualization tool of the variable being set.
- The upper and lower slider controls are set to the minimum and maximum values by default. For transparency, these are set under the range settings. Drag the upper or lower slider control up or down to adjust the distribution of the variable.
- Other controls, such as symbol order or color scheme direction, must be managed from the Primary symbology tab or in the color scheme editor.
- If the source data has been changed or updated, click the Refresh values button located below the histogram.
- By default, visual variable legends are not visible in the Contents pane or in layouts. To change this setting, check the Show legend check box.