Method 1a - Adjust Image - There is no way to easily convert such a graph. But, to see what it will look like once completed, take it into your favourite graphics program and resize it so the blocks are square. You want to make the overall image taller from top to bottom. If the resulting design is acceptable, go ahead and use the chart as is.
Method 1b - Adjust Image - There is no way to easily convert such a graph. But, to see what it will look like once completed, take it into your favourite graphics program and resize it so the blocks are rectangular (wider than they are tall). You want to make the overall image shorter from top to bottom. If the resulting design is acceptable, go ahead and use the chart as is.
Method 2 - Use Sqare Stitches - For crochet, working 2 dcs for each block will usually work. For knitting, garter stitch will often do the trick. If not, pull out your trusty knitting or crochet stitch library reference and look for stitches that are the same in width as they are in height. Try a few different ones to see which looks best.
Method 3 - Use Rectangular Stitches - In this case, you might want to try working 2 hdcs or 3 dcs for each block. Again, try it and see which looks best.
Method 4 - Adjust the Number of Stitches per Block - For filet crochet, you could try working 3 dcs (or tcs) instead of 2 dcs (or tcs) for each solid block, and reducing the number of chains by one for each open block.