  We report the magnetization, ac susceptibility, and specific heat of
optically float-zoned single crystals of Fe$_{1-x}$Co$_{x}$Si, $0.20 \leq x
\leq 0.50$. We determine the magnetic phase diagrams for all major
crystallographic directions and cooling histories. After zero-field cooling,
the phase diagrams resemble that of the archetypal stoichiometric cubic chiral
magnet MnSi. Besides the helical and conical state, we observe a pocket of
skyrmion lattice phase just below the helimagnetic ordering temperature. At the
phase boundaries between these states evidence for slow dynamics is observed.
When the sample is cooled in small magnetic fields, the phase pocket of
skyrmion lattice may persist metastably down to lowest temperatures. Taken
together with the large variation of the transition temperatures, transition
fields, and the helix wavelength as a function of composition, this hysteresis
identifies Fe$_{1-x}$Co$_{x}$Si as an ideal material for future experiments
exploring, for instance, the topological unwinding of the skyrmion lattice.
