What do you get as output if you type
which -all plot.m
/MATLAB/toolbox/matlab/timeseries/@timeseries/plot.m            % timeseries method
/MATLAB/toolbox/matlab/bigdata/@tall/plot.m                     % Shadowed tall method
/MATLAB/toolbox/matlab/graphics/math/@polyshape/plot.m          % Shadowed polyshape method
/MATLAB/toolbox/matlab/graphics/math/@digraph/plot.m            % Shadowed digraph method
/MATLAB/toolbox/matlab/graphics/math/@alphaShape/plot.m         % Shadowed alphaShape method
/MATLAB/toolbox/matlab/graphics/math/@graph/plot.m              % Shadowed graph method
/MATLAB/toolbox/bioinfo/bioinfo/@phytree/plot.m                 % Shadowed phytree method
/MATLAB/toolbox/bioinfo/bioinfo/microarray/@HeatMap/plot.m      % Shadowed HeatMap method
/MATLAB/toolbox/bioinfo/bioinfo/microarray/@clustergram/plot.m  % Shadowed clustergram method
/MATLAB/toolbox/curvefit/curvefit/@sfit/plot.m                  % Shadowed sfit method
/MATLAB/toolbox/curvefit/curvefit/@cfit/plot.m                  % Shadowed cfit method
/MATLAB/toolbox/econ/econ/@semiconjugateblm/plot.m              % Shadowed semiconjugateblm method
/MATLAB/toolbox/econ/econ/@conjugateblm/plot.m                  % Shadowed conjugateblm method
/MATLAB/toolbox/econ/econ/@customblm/plot.m                     % Shadowed customblm method
/MATLAB/toolbox/econ/econ/@mixsemiconjugateblm/plot.m           % Shadowed mixsemiconjugateblm method
/MATLAB/toolbox/econ/econ/@blm/plot.m                           % Shadowed blm method
/MATLAB/toolbox/econ/econ/@empiricalblm/plot.m                  % Shadowed empiricalblm method
/MATLAB/toolbox/econ/econ/@mixconjugateblm/plot.m               % Shadowed mixconjugateblm method
/MATLAB/toolbox/econ/econ/@diffuseblm/plot.m                    % Shadowed diffuseblm method
/MATLAB/toolbox/econ/econ/@lassoblm/plot.m                      % Shadowed lassoblm method
/MATLAB/toolbox/ident/ident/@iddata/plot.m                      % Shadowed iddata method
/MATLAB/toolbox/ident/nlident/@idnlhw/plot.m                    % Shadowed idnlhw method
/MATLAB/toolbox/ident/nlident/@idnlarx/plot.m                   % Shadowed idnlarx method
/MATLAB/toolbox/mbc/mbcdata/@cgrules/plot.m                     % Shadowed cgrules method
/MATLAB/toolbox/mbc/mbcmodels/@localmulti/plot.m                % Shadowed localmulti method
/MATLAB/toolbox/mbc/mbcmodels/@xregtransient/plot.m             % Shadowed xregtransient method
/MATLAB/toolbox/mbc/mbcmodels/@localavfit/plot.m                % Shadowed localavfit method
/MATLAB/toolbox/mbc/mbcmodels/@xregmodel/plot.m                 % Shadowed xregmodel method
/MATLAB/toolbox/mbc/mbcmodels/@xregtwostage/plot.m              % Shadowed xregtwostage method
/MATLAB/toolbox/mbc/mbcmodels/@localmod/plot.m                  % Shadowed localmod method
/MATLAB/toolbox/mbc/mbctools/@sweepset/plot.m                   % Shadowed sweepset method
/MATLAB/toolbox/mbc/mbcview/@cgdatasetnode/plot.m               % Shadowed cgdatasetnode method
/MATLAB/toolbox/mpc/mpc/@mpc/plot.m                             % Shadowed mpc method
/MATLAB/toolbox/risk/risk/@esbacktest/plot.m                    % Shadowed esbacktest method
/MATLAB/toolbox/risk/risk/@varbacktest/plot.m                   % Shadowed varbacktest method
/MATLAB/toolbox/robust/rctobsolete/robust/@frd/plot.m           % Shadowed frd method
/MATLAB/toolbox/robust/robust/@umargin/plot.m                   % Shadowed umargin method
/MATLAB/toolbox/shared/channel/rfprop/@propagationData/plot.m   % Shadowed propagationData method
/MATLAB/toolbox/shared/drivingscenario/@drivingScenario/plot.m  % Shadowed drivingScenario method
/MATLAB/toolbox/stats/bayesoptim/@BayesianOptimization/plot.m   % Shadowed BayesianOptimization method
/MATLAB/toolbox/stats/classreg/@LinearModel/plot.m              % Shadowed LinearModel method
/MATLAB/toolbox/wavelet/core/wavelet/@dtree/plot.m              % Shadowed dtree method
/MATLAB/toolbox/wavelet/core/wavelet/@wdectree/plot.m           % Shadowed wdectree method
/MATLAB/toolbox/wavelet/core/wavelet/@ntree/plot.m              % Shadowed ntree method
? Is there a function you created and that shadows the MATLAB plot function ? Then rename it.
If not, you should consult MATLAB Technical Support:
