Issue with multi-group barchart legend

[hokkanen]

Hi,

Thanks for a great project! I like it very much. However, I'm having troubles plotting the legend for a barchart with multiple groups, and there was no such example in the documentation that would have helped. The legend only shows the first name for all colors (in this case almond). Please see an example code below:

#include <cmath>         
#include <matplot/matplot.h>                                                                                                      
#include <random>                                                                                                                                                                                                                                                   int main() {                                                                                                                                                                                                                                                            
  std::vector<std::vector<double>> Y = {{2, 2, 2, 2}, {2, 5, 8, 11}, {3, 6, 9, 12}};                                                                                                                                                                                                                    
  auto f = matplot::figure(true); 
  matplot::bar(Y);                                                                                                                                                                                                                                                    
  auto lgd = matplot::legend({"almond", "cashew", "hazelnut", "macadamia"});                                                                                                                                                                                          
  //show();                                                                                                                                                                                                                                                           
  f->save("ex.jpg");                                                                                                                                                                                                                                                  
  return 0;                                                                                                                     
}  
```