Is there a way to add an empty entry to a Legend in Matplotlib?

Question

I wanted to add a blank space to a legend in a matplotib plot in python. I have an odd number of entries in the legend, and it currently looks like:

 _________________________________________________________________
|  handle1  -  label1   handle3  -  label3   handle5  -  label5  |
|  handle2  -  label2   handle4  -  label4                       |

However, the data is logically grouped in pairs with a control, so it would be better if it looked like this:

 _________________________________________________________________
|  handle1  -  label1   handle2  -  label2   handle4  -  label4  |
|                       handle3  -  label3   handle5  -  label5  |

The legend is being generated during automatically running a for loop over the data set:

for [folder1,folder2, label] in folder_list:
    parse_folder([folder1,folder2])
    color = next(colorgen)
    marker = next(markergen)
    ax1.errorbar(percent[0],percent[1], yerr=per_std, c=color, fmt=marker, label=label)
    if label == 'Flat Plate':
        print 'tripped'
        ax1.plot(np.NaN, np.NaN, '-', color='none', label=' ')

and then at the end calling

leg = ax1.legend(loc='lower left',fancybox=True,prop={'size':fontsize-2},ncol=4,bbox_to_anchor=(-0.1, -0.315))

Is there a way to insert this blank spot into the legend?

Answer 1

Farenorth's solution (given in the comments to the Op) give the cleanest answer.

Add an additional command in the position that you need the blank spot, using np.NaN for data and setting the color to none.

ax1.plot(np.NaN, np.NaN, '-', color='none', label='')

Note that this must be of the same type, as the method used to automatically generate the Legend gets different plot types sequentially, ie all plot, then all errorbar, etc., so the blank entry needs to be the same type to stay in order of the function calls.

Answer 2

You can define a fake white line

l = Line2D([0],[0],color="w")

Then plot your data and save the line/markers in variables

f = figure()
ax = f.add_subplot(111)

l1, = ax.plot(1*arange(10))
l2, = ax.plot(2*arange(10))
l3, = ax.plot(3*arange(10))
l4, = ax.plot(4*arange(10))
l5, = ax.plot(5*arange(10))

Finally, you call legend as follows

ax.legend((l1,l,l2,l3,l4,l5),("label1","","label2","label3","label4","label5"),
           loc='upper center',fancybox=True,ncol=3)

where for each line/marker is associated with a different label. At the place where you want to have a gap in your legend insert the fake white line l and associate a blank string to it.

Hope it helps.