Adding Two-Headed Arrow in map legend

I am trying to draw maps for the world using:

library(rworldmap)
library(maptools)
library(RColorBrewer)


tmp2<- dput(head(pece,10))
structure(list(iso3 = c("AUS", "AUT", "BEL", "CAN", "CHE", "CHL",
"CZE", "DEU", "DNK", "ESP"), eps_score = c(0.877343773841858,
2.68984365463257, 1.31406247615814, 1.98046875, 2.61666655540466,
NA, 1.44414067268372, 2.34257817268372, 2.89687490463257, 2.15937495231628
), gov_eff = c(1.76499999562899, 1.85666667421659, 1.74500000476837,
1.88416666785876, 1.99181815710935, 1.21499997377396, 0.865833342075348,
1.64999999602636, 2.15416664878527, 1.36833332975705), sh_va_enint =
c(13.4375638961792,
8.90904521942139, 10.368335723877, 14.0469560623169, NA, NA,
13.5679216384888, 9.67090892791748, 10.5978908538818, 8.34146690368652
), rd_in_va = c(2.17547988891602, 2.47147130966187, 2.53955459594727,
2.01138758659363, NA, NA, 1.49587619304657, 2.72330951690674,
2.5316367149353, 1.48551619052887)), datalabel = "", time.stamp = " 9 Mar
2016 17:43", .Names = c("iso3",
"eps_score", "gov_eff", "sh_va_enint", "rd_in_va"), formats = c("%9s",
"%8.0g", "%10.0g", "%9.0g", "%9.0g"), types = c(6L, 254L, 255L,
254L, 254L), val.labels = c("", "", "", "", ""), var.labels = c("",
"(mean) eps_score", "(mean) gov_eff", "(mean) sh_va_enint", "(mean)
rd_in_va"
), expansion.fields = list(c("_dta", "ReS_i", "countrycode"),
    c("_dta", "ReS_ver", "v.2"), c("_dta", "ReS_j", "year"),
    c("_dta", "ReS_str", "0"), c("_dta", "ReS_Xij", "a_"), c("_dta",
    "__JVarLab", "ACT"), c("_dta", "__XijVarLabrdd_", "(sum) rdd"
    ), c("_dta", "__XijVarLabp", "Value"), c("_dta", "__XijVarLabpop",
    "Population"), c("_dta", "__XijVarLabest_lu_f", "Source of lu"
    ), c("_dta", "__XijVarLablu", "Percentage of No Schooling"
    ), c("_dta", "__XijVarLabest_lp_f", "Source of lp"), c("_dta",
    "__XijVarLablp", "Percentage of Primary"), c("_dta", "__XijVarLablh",
    "Percentage of Tertiary"), c("_dta", "__XijVarLabest_lh_f",
    "Source of lh"), c("_dta", "__XijVarLabls", "Percentage of Secondary"
    ), c("_dta", "__XijVarLabest_ls_f", "Source of ls"), c("_dta",
    "__XijVarLabvalue", "Value"), c("_dta", "_TStvar", "year"
    ), c("_dta", "_TSpanel", "id2"), c("_dta", "_TSdelta",
"+1.0000000000000X+000"
    ), c("_dta", "_TSitrvl", "1"), c("_dta", "tis", "year"),
    c("_dta", "iis", "id2")), version = 12L, row.names = c("1",
"2", "3", "4", "5", "6", "7", "8", "9", "10"), class = "data.frame")
n <- joinCountryData2Map(pece, joinCode="ISO3", nameJoinColumn="iso3")
n <- n[-which(row.names(n)=='Antarctica'),]

# EPS
colourPalette <- rev(brewer.pal(7, "RdYlGn"))

eps <- mapCountryData(n, nameColumnToPlot="eps_score", mapTitle="EPS
Score",colourPalette=colourPalette,
                      catMethod="fixedWidth", missingCountryCol = "white",
addLegend=FALSE)
do.call(addMapLegend, c(eps, legendLabels="all", legendWidth=0.5))

Instead of adding numeric based legend, I would like to add a two-headed
arrow with some text. I would be grateful for any help. Thank you!

Sincerely,

Milu

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