How to use the pymavlink.mavutil.evaluate_expression function in pymavlink

def add_mavlink_packet(self, msg):
        '''add data to the graph'''
        mtype = msg.get_type()
        if mtype not in self.msg_types:
            return
        for i in range(len(self.fields)):
            if mtype not in self.field_types[i]:
                continue
            f = self.fields[i]
            self.values[i] = mavutil.evaluate_expression(f, self.state.master.messages)
        if self.livegraph is not None:
            self.livegraph.add_values(self.values)

def cmd_click(args):
    '''synthesise click at lat/lon; no arguments is "unclick"'''
    if len(args) == 0:
        mpstate.click(None)
        return
    if len(args) < 2:
        print("click LAT_EXPRESSION LNG_EXPRESSION")
        return
    lat = mavutil.evaluate_expression(args[0], mpstate.master().messages)
    lng = mavutil.evaluate_expression(args[1], mpstate.master().messages)
    mpstate.click((lat, lng))

if mtype not in field_types[i]:
            continue
        f = fields[i]
        if f.endswith(":2"):
            axes[i] = 2
            f = f[:-2]
        if f.endswith(":1"):
            first_only[i] = True
            f = f[:-2]
        v = mavutil.evaluate_expression(f, vars)
        if v is None:
            continue
        if args.xaxis is None:
            xv = t
        else:
            xv = mavutil.evaluate_expression(args.xaxis, vars)
            if xv is None:
                continue
        y[i].append(v)
        x[i].append(xv)

mtype = msg.get_type()
        for i in range(0, len(self.fields)):
            if mtype not in self.field_types[i]:
                continue
            f = self.fields[i]
            simple = self.simple_field[i]
            if simple is not None:
                v = getattr(vars[simple[0]], simple[1])
            else:
                v = mavutil.evaluate_expression(f, vars)
            if v is None:
                continue
            if self.xaxis is None:
                xv = t
            else:
                xv = mavutil.evaluate_expression(self.xaxis, vars)
                if xv is None:
                    continue
            try:
                v_f = float(v)
                xv_f = float(xv)
            except Exception:
                continue
            self.y[i].append(v_f)
            self.x[i].append(xv_f)

mtype = msg.get_type()
    if args.flightmode is not None and (len(modes) == 0 or modes[-1][1] != flightmode):
        modes.append((t, flightmode))
    if mtype not in msg_types:
        return
    for i in range(0, len(fields)):
        if mtype not in field_types[i]:
            continue
        f = fields[i]
        if f.endswith(":2"):
            axes[i] = 2
            f = f[:-2]
        if f.endswith(":1"):
            first_only[i] = True
            f = f[:-2]
        v = mavutil.evaluate_expression(f, vars)
        if v is None:
            continue
        if args.xaxis is None:
            xv = t
        else:
            xv = mavutil.evaluate_expression(args.xaxis, vars)
            if xv is None:
                continue
        y[i].append(v)
        x[i].append(xv)

def add_data(t, msg, msg_types, vars, fields, field_types, position_field_type):
    '''add some data'''

    mtype = msg.get_type()
    if mtype not in msg_types:
        return

    for i in range(0, len(fields)):
        if mtype not in field_types[i]:
            continue
        f = fields[i]
        v = mavutil.evaluate_expression(f, vars)
        if v is None:
            continue

        # Check if we have position or state information
        if f == mainstate_field:
            # Handle main state information
            # add_data.mainstate_current >= 0 : avoid starting a new linestring
            # when mainstate comes after the first position data in the log
            if (v != add_data.mainstate_current and
                    add_data.mainstate_current >= 0):
                add_data.new_linestring = True
            add_data.mainstate_current = v
        else:
            # Handle position information
            # make sure lon, lat, alt is saved in the correct order in
            # position_data (the same as in position_field_types)

def add_data(self, t, msg, vars):
        '''add some data'''
        mtype = msg.get_type()
        for i in range(0, len(self.fields)):
            if mtype not in self.field_types[i]:
                continue
            f = self.fields[i]
            simple = self.simple_field[i]
            if simple is not None:
                v = getattr(vars[simple[0]], simple[1])
            else:
                v = mavutil.evaluate_expression(f, vars)
            if v is None:
                continue
            if self.xaxis is None:
                xv = t
            else:
                xv = mavutil.evaluate_expression(self.xaxis, vars)
                if xv is None:
                    continue
            try:
                v_f = float(v)
                xv_f = float(xv)
            except Exception:
                continue
            self.y[i].append(v_f)
            self.x[i].append(xv_f)