For a reaction $X⟶Y,$, the graph of the product concentration (x) versus time (t) came out to be a straight line passing through the origin. Hence the graph of $\frac{-\mathrm{d}\left[\mathrm{x}\right]}{\mathrm{dt}}$ and time would be

Concentration Vs time ,straight line passing through origin(y = mx) . slope of line = k

$\frac{\mathrm{concentration} }{\mathrm{time} }=k$ it is zero order reaction

for zero order reaction rate is independent of  concentration of reacting substances The graph is straight line parallel to x-axis is obtained.