  We map the trends of elemental abundance ratios across the Galactic disk,
spanning R = 3-15 kpc and midplane distance |Z|= 0-2 kpc, for 15 elements in a
sample of 20,485 stars measured by the SDSS/APOGEE survey (O, Na, Mg, Al, Si,
P, S, K, Ca, V, Cr, Mn, Fe, Co, Ni). Adopting Mg rather than Fe as our
reference element, and separating stars into two populations based on [Fe/Mg],
we find that the median trends of [X/Mg] vs. [Mg/H] in each population are
nearly independent of location in the Galaxy. The full multi-element
cartography can be summarized by combining these nearly universal median
sequences with our measured metallicity distribution functions and the relative
proportions of the low-[Fe/Mg] (high-alpha) and high-[Fe/Mg] (low-alpha)
populations, which depend strongly on R and |Z|. We interpret the median
sequences with a semi-empirical "2-process" model that describes both the ratio
of core collapse and Type Ia supernova contributions to each element and the
metallicity dependence of the supernova yields. These observationally inferred
trends can provide strong tests of supernova nucleosynthesis calculations. Our
results lead to a relatively simple picture of abundance ratio variations in
the Milky Way, in which the trends at any location can be described as the sum
of two components with relative contributions that change systematically and
smoothly across the Galaxy. Deviations from this picture and future extensions
to other elements can provide further insights into the physics of stellar
nucleosynthesis and unusual events in the Galaxy's history.
