fix plot_z_2d

1. if x,y are not linspace it plots wrongly. One axes0 uses imshow, other pcolor
 
2.  sharex doesn't work
```
imshow(axes[0], data1, x=x, y=y, **kwargs_without_xlabel)
im = axes[1].pcolor(x, y, data2)
plt.colorbar(im, ax=axes[1]) # this line ruins the sharex
```