How do you choose the spacing between letters in a diagram?

Question

What considerations do I have to think about when I choose whether to set line spacing at 90%, 100%, 110% or 120% when I design a diagram?

Is there research about the effects that different line spacing has on the user?

Answer 1

90%, 100%, 110% or 120%?

There is no magic number. Why:

1. Default line height depends on the font-size you set, which is variable.
2. Fonts have different default line height. For ex: 14px font size will result in 14px line height with one font and 19px with another.
3. Every design has it's own, specific optimal value, which can only be defined by a human eye.

Effect of spacing

Spacing has always 1 effect: separating objects. Smaller space = objects belong together, bigger space objects belong apart. Like extra spacing between letters separates words and extra spacing between lines separates paragraphs. Decreasing or increasing letter spacing affects readability negatively in most cases, unless the font you are using is badly designed and has visible problems.

Should you increase or decrease line spacing, and if yes, how much? Well:

The optimal spacing really depends on your specific (diagram) design.

If you don't have a "feeling" of what looks most readable, ask a designer to review your content.

*Note: You mention "spacing between letters" and "line spacing". Know the difference: letter spacing is a horizontal space between characters, line spacing is a vertical space between lines.