The 'X' axis represents the cards' playing cost, and the 'Y' axis how many cards with each possible cost you have in your deck. Lands are ignored.

Colors
Symbols	25	23	21
Percent	36.0	33.0	30.0
Lands

Symbols represent the number of times each mana symbol is present on the casting cost of all the cards in this deck. Hybrid mana is counted as 0.5 for each color.
After deciding on the number of basic lands in your deck, type that number in the Lands textbox and press enter. The last row will give you a proportionally correct number of lands for each color.


Please note that this calculation does not take into account cards that you will play without paying their cost (like Veinfire Borderpost). Please take it as a general guideline, and adapt it to your deck type.