Caro Walnes, Muito obrigado pela atenção. Funcionou tudo aqui ...apenas não aparece no grafico no eixo x as datas. Se voce ou outro membro da lista puder me ajudar em mais isto Segue o script que você corrigiu x <- c("30/04/2010","13/05/2010","28/05/2010", "15/06/2010","30/06/2010","15/07/2010", "30/07/2010","15/08/2010","30/08/2010", "15/09/2010","30/09/2010","15/10/2010", "30/10/2010","15/11/2010","30/11/2010", "15/12/2010","30/12/2010","15/01/2011", "30/01/2011","15/02/2011","28/02/2011", "15/03/2011","30/03/2011","15/04/2011", "30/04/2011","15/05/2011","30/05/2011", "15/06/2011","30/06/2011","15/07/2011") y <- c(0.278, 0.068, 0.064, 0.028, 0.030, 0.024, NA, 0.025, 0.039, 0.077, 0.081, NA, 0.043, NA, 0.343, NA, 0.195, 0.090, 0.027, 0.157, 0.122, 0.069, 0.197, 0.107, 0.103, 0.154,0.164, 0.051,0.243,0.145) SDFE10 <- c(0.294,0.033,0.058,0.013,0.006,0.006,NA,0.018,0.031,0,0, NA,0.021,NA,0.435,NA,0.057,0.049,0.005,0.220,0.159,0.029, 0.194,0.114,0.109,0.116,0.187,0,0.266,0.121) # colocar num data.frame, eliminar NA, converter para data da <- data.frame(x=as.POSIXlt(x, format="%d/%m/%Y"), y=y, sd=SDFE10) da <- na.omit(da) da$x plot(y~x, data=da, ylab=expression(Concentração~de~Ferro~na~solução~do~solo~(mg~L^{-1})),xlab="Dia", pch=4, type="b", xaxt="n",ylim=c(-0.1,0.8)) with(da, arrows(x, y-sd, x, y+sd, code=3, angle=90,length=0.03)) #axis.POSIXct(side=1, at=da$x, format="%d/%m") axis.POSIXct(side=1, at=seq(min(da$x), max(da$x), by="month"), format="%d/%m") Cordialmente, Wenceslau Teixeira Embrapa Solos -- Wenceslau Teixeira wgt007@gmail.com skype: wgt007