# Representing Change with a Value Scale

A **value scale** displays incremental changes in a color’s value.

There are achromatic gradients, monochromatic gradients, and dichromatic gradients.


Information designers use value scales to show **differences within categories** such as temperature, crime rates, or voter turnout.


The infographic above uses a monochromatic scale to represent how long it will take before major cities are underwater.

## Achromatic Value Scale

Achromatic (i.e. colorless) value scales move through values from **black to gray to white**.

Use achromatic value scales to represent **sequential data**.

The map below uses an achromatic gradient to express wind speeds during Hurricane Sandy. It was created by Fernanda Viégas and Martin Wattenberg using surface wind data from the National Digital Forecast Database. Light values represent higher wind speeds while dark values represent lower wind speeds.


In assigning meaning to your scale, you will need to consider which end of the spectrum represents the **presence of a condition**, the **absence of a condition**, or a **neutral state of affairs**.

## Monochromatic Value Scale

Monochromatic value scales move from a **dark hue through midtones to white**.

The section of the map below uses a monochromatic value scales to represent building heights in Manhattan.


Use monochromatic value scales to **represent sequential data**. When using a monochromatic value scale, it is important to remember that colors have connotations. Make sure to **pick a color that facilitates your viewers’ comprehension**. For example, if you are representing a heat wave, red is an obvious choice, where the most saturated red areas represent the hottest temperatures.

Be vigilant not to choose colors that work against your message; don’t, for example, choose blue to represent areas of drought, as people associate water with blue. This could confuse your viewers.

## Dichromatic Value Scale

Dichromatic value scales **move from one hue to another**, where **each color represents one side of a binary opposition**.

The map below compares gas prices around the world to the price of gas in the United States. The mapmaker uses a red value scale to indicate prices greater than those in the United States and a green value scale to represent prices less than those in the United States. More heavily saturated reds and greens represent movement toward extreme gas prices.


Use dichromatic scales when you need to **represent data that diverges around some neutral point, separating into two distinct, or extreme camps**.

Use a neutral color in the middle, and extreme, opposing colors on the sides.

## Multichromatic Keys

While color scales discussed before were used to represent gradual changes within a single category, multichromatic keys are used to **represent qualitative data where there are many non-overlapping categories**.

For example, the map below uses five distinct colors to represent five distinct religious. The map below, published by the Washington Post, depicts the second largest religious tradition in each state. (Christianity is still the largest religious tradition in all 50 states.)


Use multichromatic keys to **represent qualitative data where there are more than two distinct categories**. Make sure that the colors you choose are easy to differentiate from one another.

## Takeaways

* **Use a value scale** to express gradual change across some category.
* **Decide upon a “thesis statement”** for your explanation, and design your value scale in a way that communicates your thesis.
* **Colors have connotations**. Pick a color that facilitates your viewers’ comprehension and avoid colors that work against your message.
* **Do not use color alone to convey your meaning**. Every color is associated with multiple meanings. Do not assume that your viewers will share your color associations.

**Achromatic and Monochromatic Scales**
* Use an achromatic or monochromatic scale to **represent sequential change in one variable**.
* Changes in value represent changes in the variable.

**Dichromatic Scales**

* Use a dichromatic scale to **represent data that diverges around some neutral point, separating into two distinct, or extreme camps**.
* Each extreme is represented by a different, vibrant color.
* Changes in each color’s value represent changes in the intensity or degree of each variable.

**Multichromatic Keys**

* Use a multichromatic keys to **represent qualitative data where there are multiple, distinct categories**.