The bars are calculated as dutchX(DAI / WETH) / market(DAI / WETH) and converted to percentage terms. A positive value means that DAI is cheaper on DutchX than on centralized exchanges and vice versa.
Note that it matters what side of the trade you are on. DAI discount is good if you are buying DAI, but not if you are selling DAI.
The lines are calculated as dutchX (DAI / WETH) and market (DAI / WETH).