ys[n_, z_] = (2/Pi)*
          Re[Sum[((-1)^(i + 1)*Sin[Pi*z*i]/i), {i, 1, n, 
                1}]];

Plot[ys[1, a], {a, 0. , 1}, AxesOrigin -> {0. , 0. }, 
      PlotRange -> {0, 1.25}]