I am looking to put a figure at the bottom of my document in a way that the caption is aligned with the bottom margin (i.e. no space between text and margin). Here's a MWE:

documentclass[12pt]{report}

usepackage[top=1.0in,hmargin=1.25in,height=9.0in,letterpaper,showframe]{geometry}

usepackage[document]{ragged2e}

usepackage{setspace}

usepackage{graphicx}

usepackage[labelfont=bf,justification=RaggedRight,singlelinecheck=off,%

           labelsep=period]{caption}

usepackage{duckuments}

usepackage{blindtext}



begin{document}

RaggedRight    

doublespace

chapter{Title of chapter}

blindtext  

begin{figure}[b]

    centering

    includegraphics[height=3cm]{example-image-duck}

    caption{This is a caption that is very long for the figure and it should be aligned with the bottom margin}

end{figure}

blindtext

blindtext

begin{figure}[t]

    centering

    includegraphics[height=3cm]{example-image-duck}

    caption{This is the caption.}

end{figure}    

end{document}

The first figure is set at the bottom, but the caption doesn't align with the margin (there is a space between the text and the margin). The one aligned at the top, however, does align to the margin, so I didn't want to mess with the padding the figure environment has.

Edit:

Here's a screenshot, I drew the redline (on an editor) to indicate the spacing I am talking about.

You can use flushbottom to get always the last line of a page aligned with the bottom of typing area.

Please see the code:

documentclass[12pt]{report}



usepackage[top=1.0in,hmargin=1.25in,height=9.0in,letterpaper,showframe]{geometry}

usepackage[document]{ragged2e}

usepackage{setspace}

usepackage{graphicx}

usepackage[%

  labelfont=bf,

  justification=RaggedRight,

  singlelinecheck=off,%

  labelsep=period

]{caption}



usepackage{duckuments}

usepackage{blindtext}





begin{document}

flushbottom % <========================================================

RaggedRight

doublespace

chapter{Title of chapter}

blindtext

begin{figure}[b]

  centering

  includegraphics[height=3cm]{example-image-duck}

  caption{This is a caption that is very long for the figure and it should be aligned with the bottom margin}

end{figure}

blindtext

blindtext

begin{figure}[t]

    centering

    includegraphics[height=3cm]{example-image-duck}

    caption{This is the caption.}

end{figure}

end{document}

and the result:

It seems that when the text is stretched to fit the page, the bottom float is pulled up. So by letting textfloatsep expand instead, one can prevent that from happening. All that is left is to remove the space below the baseline of the caption.

documentclass[12pt]{report}

usepackage[top=1.0in,hmargin=1.25in,height=9.0in,letterpaper,showframe]{geometry}

usepackage[document]{ragged2e}

usepackage{setspace}

usepackage{graphicx}

usepackage[labelfont=bf,justification=RaggedRight,singlelinecheck=off,%

           labelsep=period]{caption}

usepackage{duckuments}

usepackage{blindtext}



addtolength{textfloatsep}{0pt plus 0.01fil}



begin{document}

RaggedRight    

doublespace

chapter{Title of chapter}

blindtext  

begin{figure}[b]

    hrule

    centering

    includegraphics[height=3cm]{example-image-duck}

    caption{This is a caption that is very long for the figure and it should be aligned with the bottom margin}

    vskip-dpstrutbox%hrule

end{figure}

blindtext

blindtext

begin{figure}[t]

    centering

    includegraphics[height=3cm]{example-image-duck}

    caption{This is the caption.}

end{figure}    

end{document}