Drawing automatically updating dashed arrows in tikz

• Name the dots such that you can refer to them, e.g. dotsup and dotconv:

  node[dot,lbl={supercapacitor}{second line}](dotsup){}
  node[dot,lbl={conventional capacitor}{second line}](dotconv){}
  

• To draw a single arrow with a corner: draw[dashed,->] (dotconv) -| (dotsup);

• To draw two straight arrows, you can refer to the corner point by (dotconv-|dotsup).

  draw[dashed,->] (dotconv) -- (dotconv-|dotsup);
  draw[dashed,->] (dotconv-|dotsup) -- (dotsup);
  

documentclass{standalone}
usepackage{tikz}
usetikzlibrary{arrows.meta}

begin{document}

begin{tikzpicture}[
         > = Stealth,
dot/.style = {circle, fill, minimum size=5pt, inner sep=0pt},
  N/.style = {align=left},
every label/.style = {label distance = 0pt, N},
lbl/.style 2 args={append after command={
(tikzlastnode)node[right=5pt,inner sep=0pt] (tmp){#1}
(tmp.south west)
node[anchor=north west,font=footnotesize,inner sep=0pt]{#2}}}
                        ]
 path[local bounding box=dots] (1,3) node[dot,lbl={battery}{second line}]{}
  (2,2) node[dot,lbl={supercapacitor}{second line}](dotsup){}
  (3,1) node[dot,lbl={conventional capacitor}{second line}](dotconv){};

 draw [<->] ([yshift=1cm]dots.north-|0,0)
  |- ([xshift=5mm]dots.east|-0,0)
   node[pos=0.25, N, above,rotate=90]     {energy density (Wh/kg)}
   node[pos=0.75, N, below]    {power density (W/kg)};
 draw[dashed,->] (dotconv) -- (dotconv-|dotsup);
 draw[dashed,->] (dotconv-|dotsup) -- (dotsup);
end{tikzpicture}
end{document}

Huh, @gernot was faster (+1) ... I spend some time to rearrange your code and write comments in it, otherwise solution is similar: you need to names nodes which you like to connect with arrow and then simply draw lines between them as you wish:

documentclass[oneside,11pt]{book}

usepackage[semibold,tt=false]{libertine}
usepackage{libertinust1math}
usepackage[font={sf,small},labelsep=quad,labelfont=sc]{caption}
usepackage[subrefformat=parens]{subcaption}
usepackage{tikz}
usetikzlibrary{arrows.meta}
usepackage{siunitx}

begin{document}
    begin{figure}[!ht]
centering
begin{tikzpicture}[
         > = Stealth,
dot/.style = {circle, fill, minimum size=5pt, inner sep=0pt},
  N/.style = {align=left},
lbl/.style 2 args = {append after command={
                (tikzlastnode) node[right=5pt,inner sep=0pt] (tmp){#1}
                (tmp.south west) node[anchor=north west,font=footnotesize,inner sep=0pt]{#2}}}
                        ]
% axes
draw[<->]  (0,5) |-  (7,0)
    node[pos=0.25, N, above,rotate=90]     {energy density (Wh/kg)}
    node[pos=0.75, N, below]    {power density (Wh/kg)};
% nodes
path   (1,3) node (a) [dot,lbl={battery}{second line}]{}                   % aded name (a)
        (2,2) node (b) [dot,lbl={supercapacitor}{second line}]{}            % aded name (b)
        (3,1) node (c) [dot,lbl={conventional capacitor}{second line}]{};   % aded bane (c)
% dashed lines between nodes (c) and (b)
draw[densely dashed, ->] (c) -- (c -| b);
draw[densely dashed,->] (c -| b) -- (b);
end{tikzpicture}
    caption{Caption}
label{fig:ragone}
    end{figure}
end{document}

This is the above Zarko's answer after some simplification that may easier for beginners.

documentclass[oneside,11pt]{book}
usepackage{tikz}
usetikzlibrary{arrows.meta}
begin{document}
begin{figure}[!ht]
centering
begin{tikzpicture}[> = Stealth,
dot/.style={circle,fill=blue,minimum size=5pt,inner sep=0pt}]

draw[->] (0,0)--(7,0) node[pos=.5,below]
{energy density (Wh/kg)};
draw[->] (0,0)--(0,5) node[pos=.5,rotate=90,above]
{power density (Wh/kg)};

path   
(1,3) node[dot] (A) {}
(2,2) node[dot] (B) {}
(3,1) node[dot] (C) {};
path[nodes={right=1mm,align=left}]
(A) node{battery[-1.5mm]footnotesize second line}
(B) node{supercapacitor[-1.5mm]footnotesize second line}
(C) node{conventional capacitor[-1.5mm]footnotesize second line};

draw[densely dashed,->,magenta] (C)--(C-|B);
draw[densely dashed,->,magenta] (C-|B)--(B);
end{tikzpicture}
caption{Caption}
label{fig:ragone}
end{figure}
end{document}

Update: we can align the dot and the first text row with this minor change:

path 
(A) 
+(1mm,0) node[right]{battery}
+(1mm,-3.5mm) node[right]{footnotesize second line}
(B) 
+(1mm,0) node[right]{supercapacitor}
+(1mm,-3.5mm) node[right]{footnotesize second line}
(C)
+(1mm,0) node[right]{conventional capacitor}
+(1mm,-3.5mm) node[right]{footnotesize second line}
;

I put, simply the code done with Mathcha that I like very much...

%% Compile and read me!
documentclass[a4paper,12pt]{article}
usepackage[semibold,tt=false]{libertine}
usepackage{libertinust1math}
usepackage[font={sf,small},labelsep=quad,labelfont=sc]{caption}
usepackage[subrefformat=parens]{subcaption}
usepackage{tikz}
usepackage{siunitx}

begin{document}
begin{figure}[!h]
centering
tikzset{every picture/.style={line width=0.75pt}}     
begin{tikzpicture}[x=0.75pt,y=0.75pt,yscale=-1,xscale=1]
draw [color={rgb, 255:red, 255; green, 0; blue, 255 }  ,draw opacity=1 ] [dash pattern={on 4.5pt off 4.5pt}]  (430.47,241.04) -- (364.52,241.04) ;
draw [shift={(361.52,241.04)}, rotate = 360] [fill={rgb, 255:red, 255; green, 0; blue, 255 }  ,fill opacity=1 ][line width=0.08]  [draw opacity=0] (10.72,-5.15) -- (0,0) -- (10.72,5.15) -- (7.12,0) -- cycle    ;
draw    (240,293) -- (240,69) ;
draw [shift={(240,66)}, rotate = 450] [fill={rgb, 255:red, 0; green, 0; blue, 0 }  ][line width=0.08]  [draw opacity=0] (10.72,-5.15) -- (0,0) -- (10.72,5.15) -- (7.12,0) -- cycle    ;
draw    (240,293) -- (606.52,293) ;
draw [shift={(609.52,293)}, rotate = 180] [fill={rgb, 255:red, 0; green, 0; blue, 0 }  ][line width=0.08]  [draw opacity=0] (10.72,-5.15) -- (0,0) -- (10.72,5.15) -- (7.12,0) -- cycle    ;
draw  [fill={rgb, 255:red, 0; green, 0; blue, 0 }  ,fill opacity=1 ] (427.43,241.04) .. controls (427.43,239.36) and (428.79,238) .. (430.47,238) .. controls (432.15,238) and (433.52,239.36) .. (433.52,241.04) .. controls (433.52,242.72) and (432.15,244.09) .. (430.47,244.09) .. controls (428.79,244.09) and (427.43,242.72) .. (427.43,241.04) -- cycle ;
draw  [fill={rgb, 255:red, 0; green, 0; blue, 0 }  ,fill opacity=1 ] (298.43,161.04) .. controls (298.43,159.36) and (299.79,158) .. (301.47,158) .. controls (303.15,158) and (304.52,159.36) .. (304.52,161.04) .. controls (304.52,162.72) and (303.15,164.09) .. (301.47,164.09) .. controls (299.79,164.09) and (298.43,162.72) .. (298.43,161.04) -- cycle ;
draw  [fill={rgb, 255:red, 0; green, 0; blue, 0 }  ,fill opacity=1 ] (358.43,200.04) .. controls (358.43,198.36) and (359.79,197) .. (361.47,197) .. controls (363.15,197) and (364.52,198.36) .. (364.52,200.04) .. controls (364.52,201.72) and (363.15,203.09) .. (361.47,203.09) .. controls (359.79,203.09) and (358.43,201.72) .. (358.43,200.04) -- cycle ;
draw [color={rgb, 255:red, 255; green, 0; blue, 255 }  ,draw opacity=1 ] [dash pattern={on 4.5pt off 4.5pt}]  (361.52,241.04) -- (361.52,207.09) ;
draw [shift={(361.52,204.09)}, rotate = 450] [fill={rgb, 255:red, 255; green, 0; blue, 255 }  ,fill opacity=1 ][line width=0.08]  [draw opacity=0] (10.72,-5.15) -- (0,0) -- (10.72,5.15) -- (7.12,0) -- cycle    ;
% Text Node
draw (338,300) node [anchor=north west][inner sep=0.75pt]   [align=left] {energy density (Wh/kg)};
% Text Node
draw (207.5,252.5) node [anchor=north west][inner sep=0.75pt]  [rotate=-270] [align=left] {power density (Wh/kg)};
% Text Node
draw (309,152) node [anchor=north west][inner sep=0.75pt]   [align=left] {battery};
% Text Node
draw (308,168) node [anchor=north west][inner sep=0.75pt]   [align=left] {{footnotesize second line}};
% Text Node
draw (369,194) node [anchor=north west][inner sep=0.75pt]   [align=left] {supercapacitor};
% Text Node
draw (368,210) node [anchor=north west][inner sep=0.75pt]   [align=left] {{footnotesize second line}};
% Text Node
draw (440,233) node [anchor=north west][inner sep=0.75pt]   [align=left] {conventional capacitor};
% Text Node
draw (439,249) node [anchor=north west][inner sep=0.75pt]   [align=left] {{footnotesize second line}};
end{tikzpicture}
caption{Caption}
label{fig:ragone}
end{figure}
end{document}