  Fe$_y$Te$_{1-x}$Se$_x$, an archetypical iron-based high-temperature
superconductor with a simple structure but rich physical properties, has
attracted lots of attention because the two end compositions, Se content $x =
0$ and 1, exhibit antiferromagnetism and nematicity, respectively, making it an
ideal candidate for studying their interactions with superconductivity.
However, what is clearly lacking to date is a complete phase diagram of
Fe$_y$Te$_{1-x}$Se$_x$ as functions of its chemical compositions since phase
separation usually occurs from $x\sim 0.6$ to 0.9 in bulk crystals. Moreover,
fine control of its composition is experimentally challenging because both Te
and Se are volatile elements. Here we establish a complete phase diagram of
Fe$_y$Te$_{1-x}$Se$_x$, achieved by high-throughput film synthesis and
characterization techniques. An advanced combinatorial synthesis process
enables us to fabricate an epitaxial composition-spread Fe$_y$Te$_{1-x}$Se$_x$
film encompassing the entire Se content $x$ from 0 to 1 on a single piece of
CaF$_2$ substrate. The micro-region composition analysis and X-ray diffraction
show a successful continuous tuning of chemical compositions and lattice
parameters, respectively. The micro-scale pattern technique allows the mapping
of electrical transport properties as a function of relative Se content with an
unprecedented resolution of 0.0074. Combining with the spin patterns in
literature, we build a detailed phase diagram that can unify the electronic and
magnetic properties of Fe$_y$Te$_{1-x}$Se$_x$. Our composition-spread
Fe$_y$Te$_{1-x}$Se$_x$ films, overcoming the challenges of phase separation and
precise control of chemical compositions, provide an ideal platform for
studying the relationship between superconductivity and magnetism.
