plot: deal with fractional ticks

Closes #59

src/@dseries/plot.m

@@ -67,8 +67,12 @@ switch ndseries
         hh = plot(o.data);
     end
     axis tight;
-    id = get(gca, 'XTick');
-    Dates = strings(o.dates(id));
+    %remove fractional ticks
+    ax=gca;
+    xtick_values = ax.XTick;
+    integer_indices = fix(xtick_values) == xtick_values;
+    ax.XTick = xtick_values(integer_indices);
+    Dates = strings(o.dates(1:sum(integer_indices)));
     set(gca,'XTickLabel', Dates);
   case 2
     [o0, o1] = align(o, varargin{1});