docs: def add_line(preso, x1, y1, x2, y2, width="3pt", color="red"):
      """
      Arrow pointing up to right:
  
      context.xml:
  
  office:automatic-styles/
      <style:style style:name="gr1" style:family="graphic" style:parent-style-name="objectwithoutfill">
      <style:graphic-properties
        draw:marker-end="Arrow"
        draw:marker-end-width="0.3cm"
        draw:fill="none"
        draw:textarea-vertical-align="middle"/>
      </style:style>
  
      3pt width color red
  <style:style style:name="gr2" style:family="graphic" style:parent-style-name="objectwithoutfill">
      <style:graphic-properties
        svg:stroke-width="0.106cm"
        svg:stroke-color="#ed1c24" 
        draw:marker-start-width="0.359cm"
        draw:marker-end="Arrow"
        draw:marker-end-width="0.459cm"
        draw:fill="none"
        draw:textarea-vertical-align="middle"
        fo:padding-top="0.178cm"
        fo:padding-bottom="0.178cm"
        fo:padding-left="0.303cm"
        fo:padding-right="0.303cm"/>
      </style:style>
  
  
  
  
      ...
  
  office:presentation/draw:page
  
      <draw:line draw:style-name="gr1" draw:text-style-name="P2" draw:layer="layout" svg:x1="6.35cm" svg:y1="10.16cm" svg:x2="10.668cm" svg:y2="5.842cm"><text:p/></draw:line>
      """
      marker_end_ratio = .459 / 3  # .459cm/3pt
      marker_start_ratio = .359 / 3  # .359cm/3pt
      stroke_ratio = .106 / 3  # .106cm/3pt
  
      w = float(width[0:width.index("pt")])
      sw = w * stroke_ratio
      mew = w * marker_end_ratio
      msw = w * marker_start_ratio
      attribs = {
          "svg:stroke-width": "{}cm".format(sw),
          "svg:stroke-color": color,  # "#ed1c24",
          "draw:marker-start-width": "{}cm".format(msw),
          "draw:marker-end": "Arrow",
          "draw:marker-end-width": "{}cm".format(mew),
          "draw:fill": "none",
          "draw:textarea-vertical-align": "middle",
      }
      style = LineStyle(**attribs)
      # node = style.style_node()
      preso.add_style(style)
      line_attrib = {
          "draw:style-name": style.name,
          "draw:layer": "layout",
          "svg:x1": x1,
          "svg:y1": y1,
          "svg:x2": x2,
          "svg:y2": y2,
      }
      line_node = el("draw:line", attrib=line_attrib)
      preso.slides[-1]._page.append(line_node)