Specifically designed for decimal numbers whose boldness corresponds with it’s value, for use in data visualization:

The FatFonts technique is based on a new type of numeric typeface designed for visualization purposes that bridge the gap between numeric and visual representations. FatFonts are based on Arabic numerals but, unlike regular numeric typefaces, the amount of ink (dark pixels) used for each digit is proportional to its quantitative value. This enables accurate reading of the numerical data while preserving an overall visual context.

Fatfonts are designed so that the amount of dark pixels in a numeral character is proportional to the number it represents … This proportionality of ink is the main property of FatFonts. It allows us to create images of data where you can read the numbers, and represent tables that can be read as images.

You can find out more about the project here