Plot problems: xlim

Without any options, plot.density spreads out nicely, however, naturally, the line I want to add is not plotted since it
is well outside the range automatically determined by plot.density hence the need to add xlim however this produces
something I dont find aesthetically appealing. The plot is squeezed out into a very lean "bell" shape.

Using a broken axis is not a good solution to this problem (and there
are very few times that using a broken axis is a good idea)

It sounds like you are trying to compare a reference value to a
distribution.  To do this visually they both need to be on the same
scale, so that you can see the distance between the reference value
and the distribution.

Although it may not be aesthetically pleasing, it is true to the data.

Hadley