If you have a LATTICE layout, you can set the ROWWEIGHTS (or COLUMNWEIGHTS) option to make the ROWS (or COLUMNS) of the height you want based on the proportion between the two Y axes. If you want this to be dynamically controlled, you could use ROWWEIGHTS=dynamic, where the dynamic is the the text string providing the values, such as "0.6 0.4" not including the quotes or the parentheses. Then, you can generate this dynamic string based on the use case.
The actual axis length is always computed based on the size of the cell. There is no way to specify an axis length in GTL. If you want, you can keep the cells constant size and set a PAD value to provide more white space above the y-axis.
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