1. Change the default color setting for the colorblind.
2. default to color by the missingness on the selected variables, instead of the missingness on any variables.
3. removed the attached environment in initial_plot.
4. optimize the code for histogram, pairwise scatterplot, missingness map.
5. warning when only one variable is selected to draw the pairwise plot.
6. pcp is able for character and factor variables. Use 'uniminmax' as the scale.