Edit:
I have managed to resize the table to fit into one page, but for the life of me, I can't figure out why the first line is way off and gets printed into line two. I also don't understand why the text in first line doesn't get wrapped.
\documentclass[a4paper,12pt,twoside,BCOR=10mm]{scrbook}
\usepackage[utf8]{inputenc}
\usepackage[icelandic]{babel}
% \usepackage{t1enc}
\usepackage[T1]{fontenc}
\usepackage[dvipsnames]{xcolor}
\usepackage{graphicx,booktabs}
\usepackage[intoc]{nomencl}
\usepackage{enumerate,color}
\usepackage{url}
\usepackage{appendix}
\usepackage{float}
\usepackage{eso-pic}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{longtable}
\usepackage{acronym}
\usepackage{nicematrix}
%\usepackage[sf,normalsize]{subfigure}
\usepackage[format=plain,labelformat=simple,labelsep=colon]{caption}
\usepackage{placeins}
\usepackage{tabularx}
\usepackage{multirow}
\usepackage{adjustbox}
\usepackage{subcaption}
%\usepackage{subfig}
\usepackage{pgfplots}
\pgfplotsset{compat=newest}
\usepgfplotslibrary{external}
% Packages used for title page layout
\usepackage{tikz}
\usetikzlibrary{positioning}
\usepackage[pdfborder={0 0 0}]{hyperref}
% Blue color according to HÍ corporate design
\convertcolorspec{RGB}{16,9,159}{rgb}\tmphiblue
\definecolor{hiblue}{rgb}\tmphiblue
\setlength{\parskip}{\baselineskip}
\setlength{\parindent}{0cm}
\raggedbottom
\setkomafont{captionlabel}{\itshape}
\setkomafont{caption}{\itshape}
\setkomafont{section}{\FloatBarrier\Large}
\setcapwidth{\textwidth}
\usepackage{lmodern} % Use Latin Modern (instead of the default Computer Modern that is rendered using a bitmap font).
\usepackage{fixcmex} % To fix that Latin Modern large symbol math fonts has by default only one size: https://tex.stackexchange.com/a/621536
\usepackage{mathptmx}
\graphicspath{{figs/}} % Figures in directory figs
\usepackage[backend=biber, style=authoryear]{biblatex} % BibLaTeX used for references.
\usepackage{csquotes} % BibLaTex wants to have context sensitive quotes
\addbibresource{references.bib} % Name of *.bib file containing references
\begin{document}
\begin{table}
\centering
{\small\renewcommand{\arraystretch}{.8}
\caption{Spá um raforkunotkun, forgangsorka og skerðanleg orka. Almenn notkun að meðtöldum dreifitöpum og núverandi og samþykktir nýir stórnotendur (Rauntölur). \label{table:Orkuspá}}
\resizebox{!}{.3\paperheight}{
\begin{NiceTabular}{lcccccccccc}
\hline
\Block{3-1}{Ár} & \Block{3-2}{Úttekt frá dreifikerfinu forgangs- og skerðanleg orka með dreifitöpum} & \Block{3-2}{Úttekt frá flutningskerfinu} & \Block{3-2}{Flutningstöp} & \Block{3-2}{Úttekt frá vinnslufyrirtækjum} & \Block{3-2}{Samtals} \
& & & & & & & & & & \
& & & & & & & & & & \
& Orka GWh & Afl MW & Orka GWh & Afl MW & Orka GWh & Afl MW & Orka GWh & Afl MW & Orka GWh & Afl MW \
\hline
2015 & 3.604,74 & 618,46 & 14.356,34 & 1.685,89 & 369,56 & 57,11 & 468,32 & 75,40 & 18.798,96 & 2.383,25 \
2016* & 3.383,99 & 590,10 & 14.334,19 & 1.695,96 & 360,42 & 57,05 & 470,86 & 71,46 & 18.549,46 & 2.378,51 \
2017* & 3.519,13 & 608,86 & 14.869,95 & 1.750,87 & 373,07 & 61,49 & 476,38 & 84,74 & 19.238,53 & 2.443,61 \
\end{NiceTabular}}}
\end{table}
\end{document}
I am trying to have the long text within the cells to wrap similar to picture here below. However, the short headings should be located in the middle of the cell, which is why I have all \Block{3-*}
Unfortunately I haven't stumbled onto the correct answer for my problem, but I will keep looking.
\Block{3-1}is creating a cell spanning 3 rows and 1 column, so "Ár" would appear on the second row of the first column as expected, ditto the next entry "Úttekt frá [...]" is three rows tall so appears on the second row. It probably still leaves some overlap to be resolved with line breaking within the cell, but do you mean to have everyBlock{3-*}asBlock{1-*}? – Dai Bowen Apr 12 '23 at 13:32Block{3-*}asBlock{1-*}. I have this table in Excel, and the headings are bit of a mess. I am fine with "Ár" showing up on line 2. I was hoping that "Úttekt frá ..." would span the whole three lines, and be abowe the two columns below. Except "Ár", all the other headings have two columns below it. Maybe I just need to configure this more manually? – Ólafur Víðir Guðbjargarson Apr 12 '23 at 13:41\Blockwill span rows that already exist, so if you want the first row to span three rows, you need two rows of empty cells beneath it (otherwise it'll overlap the content with those non-empty cells). Using\Block{1-2}will get you the multi-column aspect, but you'll need something else to have the cell spanning multiple lines. I'd do this with\multicolumnand either manually line break within the cell or use a fixed-width column. – Dai Bowen Apr 12 '23 at 13:50\Block{3-2}does not wrap at all, and it looks like a hot mess. – Ólafur Víðir Guðbjargarson Apr 12 '23 at 13:54\multicolumnand\multilinewith breaklines and fixed width of column, but the text would not wrap. Did not think of\cell, will try that. – Ólafur Víðir Guðbjargarson Apr 12 '23 at 14:10