I like 'x' to represent length and 'y' to represent weight. Date allows you to look at different data sets, for comparison over long periods of time.
Here's a standard chart for largemouth bass
10" bass should weigh 10 oz.
12" should weigh 12 oz
14" is 1 lb 7 oz.
16" is 2 lb 4 oz.
18" is 3 lb 4 oz.
Plot these numbers as your standard curve, and compare your fish to these.

Teach a man to grow fish...
He can teach to catch fish...