Undefined function or variable 'update_opac'. Related to #197

[jolinos]

When do the following:

x = randn(1000,1);
nbins = 25;
h = histogram(x,nbins)

One get the following error:

Undefined function or variable 'update_opac'.

Error in plotlyfig/update (line 619)
                if (strcmp(obj.data{1, n}.type, 'bar') &&
                update_opac(length(ax)-n))

Error in plotlyfig (line 212)
                obj.update;

Error in fig2plotly (line 44)
p = plotlyfig(varargin{:});

Probably related to #197