I'm trying to put a yield curve (x axis is a unit of time) chart on a website and would like to know if there is a maximum ratio of x axis to y axis. Sometimes my charts look too long on the x axis and I would like to know if there is some basic design guideline.
Quoting from Stephen Few "Now You See It", 2009, pg 170:
[...] no single aspect ratio is always best. The choice of aspect ratio depends on what you're trying to see. It is sometimes worthwhile to experiment with the aspect ratio to see if something meaningful comes to light that wasn't noticeable before. William Cleveland took the time to test various aspect ratios and found that it is often helpful to set them so that the patterns we're focusing on have slopes that are approximately 45 degrees.
This concept is challenged here: Aspect Ratio and Banking to 45 Degrees
Basically: It depends. Do what works best.
It depends on what pattern you're trying to draw out. Lines in a graph communicate intermediate values, slope, and how fast something is changing. Too small of a ratio makes it hard to see the changes over time. Too large of a ratio is hard to judge because visual effect does not scale linearly with the change in value, so it exaggerates the trend.
This article illustrates what I'm trying to describe: How To Exaggerate Trends in Graphs