diff --git a/lex.c b/lex.c
index 80d8d0b..90e23d5 100644
--- a/lex.c
+++ b/lex.c
@@ -141,7 +141,7 @@ void lex_next(struct lexer *lex) {
 
     if (try_constant(lex, "pi", CONST_PI)) return;
     if (try_constant(lex, "e", CONST_E)) return;
-    if (try_constant(lex, "skibidi", CONST_S)) return;
+    if (try_constant(lex, "skibidi", CONST_S)) return; /* easter egg */
 
     if (try_advance_identifier(lex, lex->variable_name)) {
         lex->tok.type = TOK_VARIABLE;
diff --git a/main.c b/main.c
index 1b2b8ed..faf41c5 100644
--- a/main.c
+++ b/main.c
@@ -3,17 +3,83 @@
 #include "lex.h"
 #include "parser.h"
 #include "ps_graph.h"
+#include "error_buffer.h"
+
+static void print_usage(FILE *f, const char *name) {
+    fprintf(f, "Usage: %s <function> <output file> [<range>]\n", name);
+    fprintf(f, " <function> ..... The function to plot the graph of.\n");
+    fprintf(f, " <output file> .. Name of the output PostScript file.\n");
+    fprintf(f, " <range> ........ Ranges of the graph in the format of \"xMin:xMax:yMin:yMax\". Optional.\n");
+
+}
+
+static int read_double(const char **str, double *out) {
+    char *end;
+    *out = strtod(*str, &end);
+
+    if (*str == end)
+        return 0;
+
+    *str = end;
+    return 1;
+}
+
+#define EXPECT_DOUBLE(str, out) \
+    if (!read_double(&str, out)) \
+        return 0;
+
+#define EXPECT_CHAR(str, c) \
+    if (*str++ != c) \
+        return 0;
+
+static int parse_range(const char *str, struct graph_range *range) {
+    EXPECT_DOUBLE(str, &range->xmin);
+    EXPECT_CHAR(str, ':');
+    EXPECT_DOUBLE(str, &range->xmax);
+    EXPECT_CHAR(str, ':');
+    EXPECT_DOUBLE(str, &range->ymin);
+    EXPECT_CHAR(str, ':');
+    EXPECT_DOUBLE(str, &range->ymax);
+    EXPECT_CHAR(str, '\0');
+    return 1;
+}
+
+static enum error_code export_to_file(const struct expr_node *node, const struct graph_range *range, const char *out_name) {
+    FILE *file;
+
+    if (!(file = fopen(out_name, "w"))) {
+        fprintf(stderr, "Cannot open \"%s\" for writing!\n", out_name);
+        return ERR_INVALID_FILENAME;
+    }
+    
+    ps_export_graph(file, node, range);
+
+    fclose(file);
+    return ERR_NO_ERR;
+}
 
 int main(int argc, char *argv[]) { 
-
+    enum error_code err;
     struct parser parser;
     struct expr_node *node;
-    FILE *file;
-    struct graph_range graph;
+    struct graph_range range;
 
-    if (argc < 2) {
-        printf("Usage: %s <expression>\n", argv[0]);
-        return 1;
+    if (argc < 3) {
+        print_usage(stderr, argv[0]);
+        return ERR_INVALID_ARGS;
+    }
+
+    if (argc > 3) {
+        if (!parse_range(argv[3], &range)) {
+            fprintf(stderr, "Invalid format of ranges! The correct format is \"xMin:xMax:yMin:yMax\".\n");
+            return ERR_INVALID_ARGS;
+        }
+    }
+    else {
+        range.xmin = -10.0;
+        range.xmax = 10.0;
+        range.ymin = -10.0;
+        range.ymax = 10.0;
     }
 
     node = parser_parse(&parser, argv[1], "x");
@@ -22,23 +88,14 @@ int main(int argc, char *argv[]) {
         fprintf(stderr, "%s", parser_get_error_text(&parser));
         return parser_get_error(&parser);
     }
+    
+    err = export_to_file(node, &range, argv[2]);
 
-    graph.xmin = -10.0;
-    graph.xmax = 10.0;
-    graph.ymin = -10.0;
-    graph.ymax = 10.0;
-    graph.step = 0.01;
-
-    file = fopen("out.ps", "w");
-    ps_export_graph(file, node, &graph);
-    fclose(file);
-
-    file = fopen("graph.dot", "w");
+    /* file = fopen("graph.dot", "w");
     node_debug_print_gv(node, file);
-    fclose(file);
+    fclose(file); */
 
     node_free(node);
 
-
-    return 0;
+    return err;
 }
\ No newline at end of file
diff --git a/math_functions.c b/math_functions.c
index e38d450..a3af5f2 100644
--- a/math_functions.c
+++ b/math_functions.c
@@ -69,6 +69,10 @@ static double mf_mod(double *y, const double *args) {
 
 const struct math_function *fns_get(void) {
     static const struct math_function fns[] = {
+    /*    +--- název
+          |         +--- počet argumentů
+          |         |   +--- vyhodnocovač
+          |         |   |                   */
         { "abs",    1,  mf_fabs     },
         { "exp",    1,  mf_exp      },
         { "ln",     1,  mf_log      },
diff --git a/ps_graph.h b/ps_graph.h
index 2f2ea8f..734b424 100644
--- a/ps_graph.h
+++ b/ps_graph.h
@@ -7,7 +7,6 @@
 struct graph_range {
 	double xmin, xmax;
 	double ymin, ymax;
-	double step;
 };
 
 extern void ps_export_graph(FILE *file, const struct expr_node *node, const struct graph_range *range);