Can I set up a variable such that the line parameters are based on this variable - for example, so I can set up line width, offset, label, and label rules based on a variable, defining the variable as a particular field of data (or an expression using multiple datasets)?
For my purposes I work with traffic projections - datasets I work with typically have speed/delay/volume for AM/PM in multiple projection years (2020/2025).
A particularly useful plot I use is a bandwidth plot (similar to the size graduation style), a visual representation whereby higher road volumes are shown with wider lines - however unlike the size graduation style, a road has two directions, and as such the lines are offset from the line position so as not to overlap the opposite direction. I also work over big areas so I don't want to show low values, so I filter these out. I may also need to use one colour for an increase in traffic on a road, and a different colour for a decrease. By this stage I have set 5 or 6 style elements based on my data, and if I want to look at the delays rather than volumes I have to manually adjust each of these style elements
I'd like to make it so I can set up my style generically, say set var="2020_AM_Volume" or var="2025_PM_Volume" or var="2025_AM_Volume"-"2015_AM_Volume", and refer to 'var' throughout my style setup - is this at all achievable?