How do I set the upper and lower bounds of a dynamic graph?

Question

Say I want to display a graph of today's stock market. I gather all my data, and I go to display the graph...is there a well known formula to set the upper and lower bounds to?

Do I need to factor in all the data on the stock market from the last year to get some context? In some ways this is a design/math question, but essentially, I want to know, is there a well known formula for making graphs for things like network traffic and visitor counts intuitive to read?

Answer 1

Maybe I'm underestimating your problem, but if you just want to show absolute movement, why not do something like

factor = 0.1 # e.g.
todays_span = todays_max - todays_min
lower_bound = todays_min - factor * todays_span
upper_bound = todays_max + factor * todays_span

to fit the curve and leave some space above and below? If you need the user to see relative changes, you have to set the lower bound to zero.