title: Highlight a single point with a marker in lineplot
body: <p>I would like to highlithgt a single point on my lineplot graph using a marker. So far I managed to create my plot and insert the highlight where I wanted.</p>
  <p>The problem is that I have 4 differents lineplot (4 different categorical attributes) and I get the marker placed on every sigle lineplot like in the following image:</p>
  <p><a href="https://i.stack.imgur.com/9Caqi.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/9Caqi.png" alt="Plot" /></a></p>
  <p>I would like to place the marker only on the 2020 line (the purple one). This is my code so far:</p>
  <pre><code>import seaborn as sns
  import matplotlib.pyplot as plt
  import matplotlib.ticker as plticker
  import seaborn as sns
  import numpy as np
  import matplotlib.gridspec as gridspec
  
  fig = plt.figure(figsize=(15,10))
  gs0 = gridspec.GridSpec(2,2, figure=fig, hspace=0.2)
  
  ax1 = fig.add_subplot(gs0[0,:]) # lineplot
  ax2 = fig.add_subplot(gs0[1,0]) #Used for another plot not shown here
  ax3 = fig.add_subplot(gs0[1,1]) #Used for another plot not shown here
  
  flatui = [&quot;#636EFA&quot;, &quot;#EF553B&quot;, &quot;#00CC96&quot;, &quot;#AB63FA&quot;]
  
  sns.lineplot(ax=ax1,x=&quot;number of weeks&quot;, y=&quot;avg streams&quot;, hue=&quot;year&quot;, data=df, palette=flatui, marker = 'o', markersize=20, fillstyle='none', markeredgewidth=1.5, markeredgecolor='black', markevery=[5])
  
  ax1.yaxis.set_major_formatter(ticker.FuncFormatter(lambda x, pos: '{:,.0f}'.format(x/1000) + 'K'))
  
  ax1.set(title='Streams trend')
  ax1.xaxis.set_major_locator(ticker.MultipleLocator(2))
  </code></pre>
  <p>I used the <code>markevery</code> field to place a marker in position 5. Is there a way to specify also on which line/category place my marker?</p>
  <p><strong>EDIT</strong>: This is my dataframe:</p>
  <pre><code>    avg streams date    year    number of weeks
  0   145502.475  01-06   2017    0
  1   158424.445  01-13   2017    1
  2   166912.255  01-20   2017    2
  3   169132.215  01-27   2017    3
  4   181889.905  02-03   2017    4
  ... ... ... ... ...
  181 760505.945  06-26   2020    25
  182 713891.695  07-03   2020    26
  183 700764.875  07-10   2020    27
  184 753817.945  07-17   2020    28
  185 717685.125  07-24   2020    29
  
  186 rows × 4 columns
  </code></pre>