Here is a link to Moore Research that has all the long term commodity charts going back to the early 70's for free. This site has more info than you would ever need for commodities trading including high odds set ups at specific times to enter and exit. Much of it is subscription info but there is still a lot of free stuff there. Most books with commodity charts use their data. This wont show the exact price difference between Corn and Wheat historically I don't think but will give you an idea if you print off both charts and compare.