--# you may as well use numbers instead of strings
local mfsu = {
  peripheral.wrap("mfsu_3");
  peripheral.wrap("mfsu_4");
  peripheral.wrap("mfsu_5");
  peripheral.wrap("mfsu_6");
  peripheral.wrap("mfsu_7");
  peripheral.wrap("mfsu_8");
  peripheral.wrap("mfsu_9");
  peripheral.wrap("mfsu_10");
  peripheral.wrap("mfsu_11");
}

local m = peripheral.wrap("monitor_13")
m.clear()
local storedEU = {}
local storedPerc = {}
local plotter = {}
local printer = {}
local energyTotals = 0
local energyFlow = 0

local function shorten(num)
  if num then
    return string.format("%.1f", num)
  end
  return "0"
end

local function colorPerc(num)
  num = tonumber(num)
  if num then --# you had ~= nil here
    --# order has been switched so elseif can be used
    if num > 90 then
      m.setTextColor(colors.lime)
    elseif num > 60 then
      m.setTextColor(colors.yellow)
    elseif num > 30 then
      m.setTextColor(colors.orange)
    else
      m.setTextColor(colors.red)
    end
    m.write(num.."%")
    space = #tostring(num) - 5 * (-1) --# string.len and # are the same on strings
    for i = 1, space do
      m.write(" ")
    end
    m.setTextColor(colors.white)
  end
end

local function storedTotal()
  local energyTotal = 0
  for i = 1,9 do
    energyTotal = energyTotal + mfsu[i].getEUStored() --# removed tostring 'cause the table is now indexes instead of keys
  end
  energyFlow = (energyTotal - energyTotals) / 20
  energyTotals = energyTotal
  energyPercentage = shorten(energyTotal / 360000000 * 100) --# you had tostring on this
  m.setCursorPos(3,3)
  m.write("Total Energy Stored: "..energyTotal.." EU")
  m.setCursorPos(9,4)
  m.write("Total Capacity: ")
  colorPerc(energyPercentage)
end

local function storedEach()
  for i = 1,9 do
    storedEU[i] = mfsu[i].getEUStored() --# removed tostring 'cause the table is now indexes instead of keys
    storedPerc[i] = shorten(storedEU[i] / 40000000 * 100) --# you had tostring on this
  end
  m.setCursorPos(1,6)
  for i = 1,3 do
    m.write("MFSU"..i..": ")
    colorPerc(storedPerc[i])
    xPos = i * 14
    m.setCursorPos(xPos,6)
  end
  m.setCursorPos(1,7)
  for i = 4,6 do
    m.write("MFSU"..i..": ")
    colorPerc(storedPerc[i])
    xPos = (i-3) * 14
    m.setCursorPos(xPos,7)
  end
  m.setCursorPos(1,8)
  for i = 7,9 do
    m.write("MFSU"..i..": ")
    colorPerc(storedPerc[i])
    xPos = (i-6) * 14
    m.setCursorPos(xPos,8)
  end
end

local function flowChart()
  for i = 1,15 do
    m.setCursorPos(6,(i + 10))
    m.write("|")
  end
  for i = 1,39 do
    m.setCursorPos(i,26)
    m.write("-")
  end
  for i = 1,15 do
    m.setCursorPos(34,(i + 10))
    m.write("|")
  end
  for i = 1,2 do
    if i == 1 then
      yP = 1
    else
      yP = 35
    end
    m.setCursorPos(yP,11)
    m.write("2.1k ")
    m.setCursorPos(yP,13)
    m.write("1.5k ")
    m.setCursorPos(yP,15)
    m.write(" 900 ")
    m.setCursorPos(yP,17)
    m.write(" 300 ")
    m.setCursorPos(yP,19)
    m.write("-300 ")
    m.setCursorPos(yP,21)
    m.write("-900 ")
    m.setCursorPos(yP,23)
    m.write("-1.5k")
    m.setCursorPos(yP,25)
    m.write("-2.1k")
  end
end

local function plotPos(num)
  local plotter,plotW,plotR = energyFlow / 300,0,0
  if plotter < 0 then
    plotW = tonumber(string.sub(tostring(plotter),2,2))
    plotR = tonumber(string.sub(tostring(plotter),4,4))
    negative = true
  else
    plotW = tonumber(string.sub(tostring(plotter),1,1))
    plotR = tonumber(string.sub(tostring(plotter),3,3))
    negative = false
  end
  if plotR then
    if plotR >= 5 then
      plotW = plotW + 1
    end
  end
  if not negative then --# you had == false here
    plotW = plotW * (-1)
  end
  return plotW
end

local function drawPlots()
  m.setCursorPos(12,26)
  m.write("--("..energyFlow.." EU/t)--------")
  yPos = plotPos(energyFlow) + 18
  printer[34] = yPos
  for i = 7,33 do
    j = i + 1
    h = i
    printer[i] = printer[j]
    for i = 11,25 do
      m.setCursorPos(h,i)
      if i == printer[h] then
        if i < 18 then
          m.setBackgroundColor(colors.cyan)
        else
          m.setBackgroundColor(colors.blue)
        end
      else
        m.setBackgroundColor(colors.black)
      end
      m.write(" ")
      m.setBackgroundColor(colors.black)
    end
  end
end

local function label()
  m.setCursorPos(13,1)
  m.write("Energy Monitor")
  m.setCursorPos(1,10)
  m.write("---------- Energy Flow (EU/t)----------")
end

label()
flowChart()

while true do
  storedTotal()
  storedEach()
  drawPlots()
  sleep(1)
end