Expose save_png in save

[hoxbro]

Fixes #3005

save_png.mp4

Code

from io import BytesIO

import hvplot.pandas
import panel as pn
import pandas as pd
import param

pn.extension(sizing_mode="stretch_width")
ACCENT_COLOR = "#0072B5"


class Explorer(param.Parameterized):
    df = pd.DataFrame({"y": range(10)})

    def __init__(self, **params):
        super().__init__(**params)
        self.plot = pn.Column(
            self.df.hvplot().opts(color=ACCENT_COLOR, line_width=6, responsive=True),
            self.df.hvplot().opts(color=ACCENT_COLOR, line_width=6, responsive=True),
        )
        self.save_to_png = pn.widgets.FileDownload(
            filename="timeseries.png",
            label="Download .png",
            button_type="primary",
            callback=self.download_column,
        )

    def download_column(self):
        self.save_to_png.loading = True

        b = BytesIO()
        self.plot.save(b, as_png=True)
        b.seek(0)

        self.save_to_png.loading = False
        return b


explorer = Explorer()

pn.template.FastListTemplate(
    site="Awesome Panel",
    title="Application with .png download",
    sidebar=[explorer.save_to_png],
    main=[explorer.plot],
    accent_base_color=ACCENT_COLOR,
    header_background=ACCENT_COLOR,
).servable()

I don't know if this should also be made available for saving the template, right here:

panel/panel/template/base.py

Lines 247 to 249 in 5bd9fc8

	def save(self, filename, title=None, resources=None, embed=False,
	max_states=1000, max_opts=3, embed_json=False,
	json_prefix='', save_path='./', load_path=None):

I have observed that sometimes the saved plot is cropped, but I don't think this is related to the changes in the PR.
Cropped

Not cropped

[codecov]

Codecov Report

Merging #3155 (a87c669) into master (e8746ef) will decrease coverage by 0.00%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #3155      +/-   ##
==========================================
- Coverage   83.00%   83.00%   -0.01%     
==========================================
  Files         192      192              
  Lines       25477    25483       +6     
==========================================
+ Hits        21147    21151       +4     
- Misses       4330     4332       +2     

Impacted Files	Coverage Δ
panel/viewable.py	70.44% <ø> (ø)
panel/io/save.py	62.60% <66.66%> (+0.32%)	⬆️
panel/io/state.py	68.83% <0.00%> (-0.25%)	⬇️
panel/widgets/base.py	94.73% <0.00%> (+0.07%)	⬆️

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[philippjfr]

Thanks @hoxbro!