Prevent labels from overlapping in matplotlib vertical timeline plot

Question

The methods I normally use for prevent overlap of data labels don't seem to apply to this matplotlib-based vertical timeline. Normally I would use something like autofmt_xdate .

I don't actually care how "tall" (long) the chart needs to be or if the vertical distance between the items perfectly preserves the ratio of the date gap between items. I just don't want the text labels to overlap.

I am not very experience with matplotlib but thusfar I've determined that the code that controls the text labels is

_ = ax.text(label_offsets[i], d, l, ha=align, fontfamily='serif', fontweight='bold', color='royalblue',fontsize=12)

where d is a timestamp being used as a y-axis coordinate. I tried creating a loop counter and a list to contain the value of d from each loop iteration, so that I could make a comparison between the current and prior date, then add a fixed value (eg 15 days) to the current value of d if the prior value of d was too close. That didn't seem to do the trick.

The plot code:

chartdata = pd.read_csv(
    "data/Events.csv"
)

chartdata = chartdata.query('Year > 1939')

dates = pd.to_datetime(chartdata['Date_Clean_Approx'])
min_date = date(np.min(dates).year - 2, np.min(dates).month, np.min(dates).day)
max_date = date(np.max(dates).year + 2, np.max(dates).month, np.max(dates).day)

labels = chartdata['Name']

# labels with associated dates
labels = ['{0:%d %b %Y}:\n{1}'.format(d, l) for l, d in zip (labels, dates)]

fig, ax = plt.subplots(figsize=(6, 32), constrained_layout=True)
_ = ax.set_xlim(-20, 20)
_ = ax.set_ylim(min_date, max_date)
_ = ax.axvline(0, ymin=0.05, ymax=0.95, c='deeppink', zorder=1)

_ = ax.scatter(np.zeros(len(dates)), dates, s=120, c='palevioletred', zorder=2)
_ = ax.scatter(np.zeros(len(dates)), dates, s=30, c='darkmagenta', zorder=3)

label_offsets = np.repeat(2.0, len(dates))
label_offsets[1::2] = -2.0

for i, (l, d) in enumerate(zip(labels, dates)):
    d = d - timedelta(days=90)
    align = 'right'
    if i % 2 == 0:
        align = 'left'
    _ = ax.text(label_offsets[i], d, l, ha=align, fontfamily='serif', fontweight='bold', color='royalblue',fontsize=12)

stems = np.repeat(2.0, len(dates))
stems[1::2] *= -1.0    
x = ax.hlines(dates, 0, stems, color='darkmagenta')

# hide lines around chart
for spine in ["left", "top", "right", "bottom"]:
    _ = ax.spines[spine].set_visible(False)

# hide tick labels
_ = ax.set_xticks([])
_ = ax.set_yticks([])
_ = ax.set_title('UAP (UFO) Milestones, 1940 - Present', 
                 fontweight="bold", 
                 fontfamily='serif', 
                 fontsize=16, 
                 color='royalblue')

pyplot(fig)

As you can see in the image, I increased the height of the plot to reduce overlap in the labels. The plot got very long but still has overlap in text. Once a solution is determined I think that embedding the logic to "adjust relative gap size and chart height as needed to avoid text label overlap" into a convenient plot function would be a large contribution to the matplotlib library.

Answer 1

I'm definitely open to better, more programmatic, solutions but for now I replaced the dates with a same-length vector of consecutive integers and used it in place of the actual dates everywhere except within the text of the labels.

###### Timeline#
chartdata = pd.read_csv(
    "/home/kodachi/Documents/ET/aliendb/www/app/data/Events.csv"
)

chartdata=chartdata.query('Year > 1939')
dates = pd.to_datetime(chartdata['Date_Clean_Approx'])
min_date = date(np.min(dates).year - 2, np.min(dates).month, np.min(dates).day)
max_date = date(np.max(dates).year + 2, np.max(dates).month, np.max(dates).day)

###
# fake date 
fake_d=np.c_[1:len(dates)]
###
labels = chartdata['Name']

# labels with associated dates
labels = ['{0:%d %b %Y}:\n{1}'.format(d, l) for l, d in zip (labels, dates)]

fig, ax = plt.subplots(figsize=(8, 28))#, constrained_layout=True)
_ = ax.set_xlim(-20, 20)
#_ = ax.set_ylim(min_date, max_date)
_ = ax.set_ylim(1, 96)
_ = ax.axvline(0, ymin=0.05, ymax=.985, c='deeppink', zorder=1)#ymax=0.95
#_ = ax.scatter(np.zeros(len(dates)), dates, s=120, c='palevioletred', zorder=2)
#_ = ax.scatter(np.zeros(len(dates)), dates, s=30, c='darkmagenta', zorder=3)
_ = ax.scatter(np.zeros(len(fake_d)), fake_d, s=120, c='palevioletred', zorder=2)
_ = ax.scatter(np.zeros(len(fake_d)), fake_d, s=30, c='darkmagenta', zorder=3)

#label_offsets = np.repeat(2.0, len(dates))
label_offsets = np.repeat(2.0, len(fake_d))
label_offsets[1::2] = -2.0


for i, (l, d) in enumerate(zip(labels, fake_d)): #dates
    #d = d - timedelta(days=90) 
    align = 'right'
    if i % 2 == 0:
        align = 'left'
    _ = ax.text(label_offsets[i], d, l, ha=align, fontfamily='serif', 
                fontweight='bold', color='royalblue',fontsize=12)

#stems = np.repeat(2.0, len(dates))
stems = np.repeat(2.0, len(fake_d))
stems[1::2] *= -1.0    
#x = ax.hlines(dates, 0, stems, color='darkmagenta')
x = ax.hlines(fake_d, 0, stems, color='darkmagenta')

# hide lines around chart
for spine in ["left", "top", "right", "bottom"]:
    _ = ax.spines[spine].set_visible(False)

# hide tick labels
_ = ax.set_xticks([])
_ = ax.set_yticks([])
_ = ax.set_title('UAP (UFO) Milestones, 1940 - Present', 
                 fontweight="bold", 
                 fontfamily='serif', 
                 fontsize=16, 
                 color='darkgreen')