axonometric grid lacks ratio property

It's hard to picture what you want.
Please, detail what do you mean with "scale for each axe". Make a graphical representation.

Closing by lack of reactions.

I'd like to second that request (and hope that it gets reopened).

What I would like to have is a way to tell inkscape to not space the axonometric grid lines according to its calculations from the angles and the one spacing parameter, but to give individual distances in all three directions along the grid axes. The current mode of operation seems to try to keep the axis scalings in some smart relation (I couldn't find any documentation about what precisely it tries to do).

When constructing axonometric projections it is quite common to use standard angles and also predefined length ratios for the different axes that are not precisely the theoretical ideal values [1]. With the current implementation that is not possible. One example would be the German "Ingenieursprojektion" specified in DIN 5 or ISO 5456-3 [0]. But there are others as well where simplified length scalings are used.

[0] Sorry, nothing in the English Wikipedia. https://de.wikipedia.org/wiki/Dimetrie
[1] Again no English link since the German Wikipedia is way better with these things. Sorry. https://de.wikipedia.org/wiki/Axonometrie

After thinking about this a little more I fear that it is not possible to achieve. I tried to draw a small graphics showing the problem. Red, Green and Blue are the axes of the system, each representing 1 real world unit in length. In the drawing Red and Green are of length 1 while Blue is shortened to 0.5. Together with the angles of 7° and 41.5° this is exactly the aforementioned Ingenieursprojektion (for the sake of the example). The magenta lines are the grid lines that a user would like to have available in order to easily construct his projected object. Line 1 is telling him that Red has length 1, line 6 tells that Green has 1 and line 3 allows to construct Blue of length 0.5. However, since the lines 3 would be duplicated upwards (bad explanation, I hope you know what I mean) you would get parallel lines for the short distance and others for the long distance (i.e., the parallels of line 3 and those of line 1) that are in the general case unevenly spaced and making the whole thing extremely hard to understand at best.

So instead I would like to see two enhancements:
1) When drawing straight lines the angle locking with ctrl should also lock to the grid angles. That way a line parallel to the grid but not exactly on it could be drawn.
2) And as a second part to solve the problem at hand we would need a way to draw a line by clicking on the start point and telling Inkscape the desired length. Currently that length is already shown in the status bar. A second click would then only determine the angle (that could be locked to the grid angle) and would allow to precisely draw axonometric drawings.

I am not sure how to proceed from here, filing a new feature wish? Discussing it on the mailing list or in IRC first? Or renaming this one? I'd be happy to give feedback, but I am a little short of time so I won't be able to write any code.